feat: add message trace propagation across servers (Gap 10.4)

Add TraceContextPropagator and TraceContext to Internal/MessageTraceContext.cs
for Nats-Trace-Parent header injection and extraction across server hops.
Also add ConnzSortOption/ConnzSorter stubs to fix pre-existing build errors
in Monitoring/Connz.cs. Covered by 10 new tests in TraceContextPropagationTests.cs.

src/NATS.Server/Internal/MessageTraceContext.cs

@@ -671,6 +671,196 @@ public sealed class MsgTraceContext
     }
 }
 
+/// <summary>
+/// Immutable trace context for a single server hop.
+/// Go reference: server/msgtrace.go — trace context embedding and extraction.
+/// </summary>
+public sealed record TraceContext(
+    string TraceId,
+    string SpanId,
+    string? Destination,
+    bool TraceOnly,
+    DateTime CreatedAt);
+
+/// <summary>
+/// Manages message trace context propagation across server hops.
+/// Handles the Nats-Trace-Parent header used to correlate trace spans
+/// as a message travels through a NATS cluster.
+/// Go reference: server/msgtrace.go — trace context embedding and extraction.
+/// </summary>
+public sealed class TraceContextPropagator
+{
+    /// <summary>Header carrying the traceparent in the form {traceId}-{spanId}.</summary>
+    public const string TraceParentHeader = "Nats-Trace-Parent";
+
+    /// <summary>Header carrying the trace destination subject.</summary>
+    public const string TraceDestHeader = "Nats-Trace-Dest";
+
+    /// <summary>Header that, when present, suppresses message delivery.</summary>
+    public const string TraceOnlyHeader = "Nats-Trace-Only";
+
+    private static readonly byte[] CrLf = "\r\n"u8.ToArray();
+    private static readonly byte[] HeaderSep = ": "u8.ToArray();
+
+    /// <summary>
+    /// Creates a new trace context for an origin message.
+    /// </summary>
+    public static TraceContext CreateTrace(string traceId, string spanId, string? destination = null)
+        => new(traceId, spanId, destination, TraceOnly: false, DateTime.UtcNow);
+
+    /// <summary>
+    /// Extracts a trace context from raw NATS message headers.
+    /// Parses "Nats-Trace-Parent: {traceId}-{spanId}" from the header block.
+    /// Returns null if the header is absent or malformed.
+    /// </summary>
+    public static TraceContext? ExtractTrace(ReadOnlySpan<byte> headers)
+    {
+        if (headers.IsEmpty)
+            return null;
+
+        // Skip the NATS/1.0 status line
+        var crlf = CrLf.AsSpan();
+        int firstCrlf = headers.IndexOf(crlf);
+        if (firstCrlf < 0)
+            return null;
+
+        int i = firstCrlf + 2;
+        string? traceParent = null;
+        string? destination = null;
+        bool traceOnly = false;
+
+        while (i < headers.Length)
+        {
+            // Find colon
+            int colonIdx = -1;
+            for (int j = i; j < headers.Length; j++)
+            {
+                if (headers[j] == (byte)':')
+                {
+                    colonIdx = j;
+                    break;
+                }
+                if (headers[j] == (byte)'\r' || headers[j] == (byte)'\n')
+                    break;
+            }
+
+            if (colonIdx < 0)
+            {
+                int nextCrlf = headers[i..].IndexOf(crlf);
+                if (nextCrlf < 0) break;
+                i += nextCrlf + 2;
+                continue;
+            }
+
+            var keySpan = headers[i..colonIdx];
+            i = colonIdx + 1;
+
+            // Skip whitespace
+            while (i < headers.Length && (headers[i] == (byte)' ' || headers[i] == (byte)'\t'))
+                i++;
+
+            int valStart = i;
+            int valCrlf = headers[valStart..].IndexOf(crlf);
+            if (valCrlf < 0) break;
+
+            int valEnd = valStart + valCrlf;
+            while (valEnd > valStart && (headers[valEnd - 1] == (byte)' ' || headers[valEnd - 1] == (byte)'\t'))
+                valEnd--;
+
+            if (keySpan.Length > 0)
+            {
+                var key = Encoding.ASCII.GetString(keySpan);
+                var val = Encoding.ASCII.GetString(headers[valStart..valEnd]);
+
+                if (key.Equals(TraceParentHeader, StringComparison.OrdinalIgnoreCase))
+                    traceParent = val;
+                else if (key.Equals(TraceDestHeader, StringComparison.OrdinalIgnoreCase))
+                    destination = val;
+                else if (key.Equals(TraceOnlyHeader, StringComparison.OrdinalIgnoreCase))
+                    traceOnly = val is "1" or "true" or "on";
+            }
+
+            i = valStart + valCrlf + 2;
+        }
+
+        if (traceParent == null)
+            return null;
+
+        // Parse "{traceId}-{spanId}": split on the first dash. Callers must use
+        // dash-free identifiers (e.g. hex-encoded bytes) to avoid ambiguity.
+        int dash = traceParent.IndexOf('-');
+        if (dash < 0 || dash == traceParent.Length - 1)
+            return null;
+
+        var traceId = traceParent[..dash];
+        var spanId = traceParent[(dash + 1)..];
+
+        return new TraceContext(traceId, spanId, destination, traceOnly, DateTime.UtcNow);
+    }
+
+    /// <summary>
+    /// Injects a trace context into a header block, appending
+    /// "Nats-Trace-Parent: {traceId}-{spanId}\r\n".
+    /// If existingHeaders is empty a minimal NATS/1.0 header block is created.
+    /// </summary>
+    public static byte[] InjectTrace(TraceContext context, ReadOnlySpan<byte> existingHeaders)
+    {
+        var headerLine = $"{TraceParentHeader}: {context.TraceId}-{context.SpanId}\r\n";
+        var headerBytes = Encoding.ASCII.GetBytes(headerLine);
+
+        if (existingHeaders.IsEmpty)
+        {
+            // Build minimal NATS/1.0 block
+            var preamble = Encoding.ASCII.GetBytes("NATS/1.0\r\n");
+            var result = new byte[preamble.Length + headerBytes.Length + 2];
+            preamble.CopyTo(result, 0);
+            headerBytes.CopyTo(result, preamble.Length);
+            "\r\n"u8.CopyTo(result.AsSpan(preamble.Length + headerBytes.Length));
+            return result;
+        }
+
+        // Append before the terminal \r\n (if present)
+        var existing = existingHeaders.ToArray();
+
+        // Strip trailing \r\n if present so we can append cleanly
+        int insertAt = existing.Length;
+        if (insertAt >= 2
+            && existing[insertAt - 2] == (byte)'\r'
+            && existing[insertAt - 1] == (byte)'\n')
+        {
+            insertAt -= 2;
+        }
+
+        var final = new byte[insertAt + headerBytes.Length + 2];
+        existing.AsSpan(0, insertAt).CopyTo(final);
+        headerBytes.CopyTo(final, insertAt);
+        final[insertAt + headerBytes.Length] = (byte)'\r';
+        final[insertAt + headerBytes.Length + 1] = (byte)'\n';
+        return final;
+    }
+
+    /// <summary>
+    /// Creates a child span that preserves the parent TraceId but
+    /// uses a new SpanId for this hop.
+    /// </summary>
+    public static TraceContext CreateChildSpan(TraceContext parent, string newSpanId)
+        => new(parent.TraceId, newSpanId, parent.Destination, parent.TraceOnly, DateTime.UtcNow);
+
+    /// <summary>
+    /// Returns true if the header block contains a Nats-Trace-Parent header,
+    /// indicating the message should be traced.
+    /// </summary>
+    public static bool ShouldTrace(ReadOnlySpan<byte> headers)
+    {
+        if (headers.IsEmpty)
+            return false;
+
+        // Fast path: search for the header name as an ASCII byte sequence
+        var needle = Encoding.ASCII.GetBytes(TraceParentHeader + ":");
+        return headers.IndexOf(needle.AsSpan()) >= 0;
+    }
+}
+
 /// <summary>
 /// JSON serialization context for message trace types.
 /// </summary>

src/NATS.Server/Monitoring/Connz.cs

@@ -188,6 +188,26 @@ public sealed record ConnzFilterResult(
     int Offset,
     int Limit);
 
+/// <summary>
+/// Sort options for the /connz endpoint.
+/// Go reference: monitor.go ConnzSortOpt constants.
+/// </summary>
+public enum ConnzSortOption
+{
+    ConnectionId,
+    Start,
+    Subs,
+    Pending,
+    MsgsTo,
+    MsgsFrom,
+    BytesTo,
+    BytesFrom,
+    LastActivity,
+    Uptime,
+    Idle,
+    RTT,
+}
+
 /// <summary>
 /// Query-string options for the account-scoped filter API.
 /// Parses the ?acc=, ?state=, ?offset=, and ?limit= parameters that the Go server
@@ -206,7 +226,20 @@ public sealed class ConnzFilterOptions
     public int Limit { get; init; } = 1024;
 
     /// <summary>
-    /// Parses a raw query string (e.g. "?acc=ACCOUNT&amp;state=open&amp;offset=0&amp;limit=100")
+    /// Sort field for connection listing. Default: <see cref="ConnzSortOption.ConnectionId"/>.
+    /// Go reference: monitor.go ConnzOptions.SortBy.
+    /// </summary>
+    public ConnzSortOption SortBy { get; init; } = ConnzSortOption.ConnectionId;
+
+    /// <summary>
+    /// When <see langword="true"/>, reverses the natural sort direction for the chosen
+    /// <see cref="SortBy"/> option.
+    /// Go reference: monitor.go ConnzOptions.SortBy (descending variant).
+    /// </summary>
+    public bool SortDescending { get; init; }
+
+    /// <summary>
+    /// Parses a raw query string (e.g. "?acc=ACCOUNT&amp;state=open&amp;offset=0&amp;limit=100&amp;sort=bytes_to")
     /// into a <see cref="ConnzFilterOptions"/> instance.
     /// </summary>
     public static ConnzFilterOptions Parse(string? queryString)
@@ -221,6 +254,8 @@ public sealed class ConnzFilterOptions
         string? stateFilter = null;
         int offset = 0;
         int limit = 1024;
+        var sortBy = ConnzSortOption.ConnectionId;
+        bool sortDescending = false;
 
         foreach (var pair in qs.Split('&', StringSplitOptions.RemoveEmptyEntries))
         {
@@ -244,6 +279,12 @@ public sealed class ConnzFilterOptions
                 case "limit" when int.TryParse(value, out var l):
                     limit = l;
                     break;
+                case "sort":
+                    sortBy = ConnzSorter.Parse(value);
+                    break;
+                case "desc" when value is "1" or "true":
+                    sortDescending = true;
+                    break;
             }
         }
 
@@ -253,6 +294,8 @@ public sealed class ConnzFilterOptions
             StateFilter = stateFilter,
             Offset = offset,
             Limit = limit,
+            SortBy = sortBy,
+            SortDescending = sortDescending,
         };
     }
 }
@@ -371,3 +414,24 @@ public sealed class ConnzOptions
 
     public int Limit { get; set; } = 1024;
 }
+
+/// <summary>
+/// Parses sort query-string values for the /connz endpoint.
+/// Go reference: monitor.go ConnzSortOpt string constants.
+/// </summary>
+public static class ConnzSorter
+{
+    public static ConnzSortOption Parse(string value) => value.ToLowerInvariant() switch
+    {
+        "start" or "start_time" => ConnzSortOption.Start,
+        "subs" => ConnzSortOption.Subs,
+        "pending" => ConnzSortOption.Pending,
+        "msgs_to" or "msgs_from" => ConnzSortOption.MsgsTo,
+        "bytes_to" or "bytes_from" => ConnzSortOption.BytesTo,
+        "last" or "last_activity" => ConnzSortOption.LastActivity,
+        "uptime" => ConnzSortOption.Uptime,
+        "idle" => ConnzSortOption.Idle,
+        "rtt" => ConnzSortOption.RTT,
+        _ => ConnzSortOption.ConnectionId,
+    };
+}