test+docs: serialise data-connection gauge tests, add metrics instrument reference (ScadaBridge#99)

DataConnectionGaugeTests and ExtendedInstrumentTests both mutate the process-global data
connection registry; put them in a non-parallel xUnit collection. Add docs/operations/metrics-reference.md.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Observability/DataConnectionGaugeCollection.cs

@@ -0,0 +1,15 @@
+namespace ZB.MOM.WW.ScadaBridge.Commons.Tests.Observability;
+
+/// <summary>
+/// Serialises every test class that mutates the process-global data-connection gauge
+/// registry in <c>ScadaBridgeTelemetry</c> (Gitea #99). The delta-against-baseline assertions in
+/// <c>DataConnectionGaugeTests</c> are only valid if no other test adds or removes a connection
+/// between the baseline read and the assertion; <c>DisableParallelization</c> makes this
+/// collection run alone, not concurrently with any other collection in the assembly.
+/// </summary>
+[CollectionDefinition(Name, DisableParallelization = true)]
+public sealed class DataConnectionGaugeCollection
+{
+    /// <summary>Collection name shared by the classes that touch the gauge registry.</summary>
+    public const string Name = "DataConnectionGauge";
+}

tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Observability/DataConnectionGaugeTests.cs

@@ -16,8 +16,11 @@ namespace ZB.MOM.WW.ScadaBridge.Commons.Tests.Observability;
 /// gauges is a process-global static, so absolute assertions would inherit the register
 /// row 32 flake that makes <c>QueueDepthGaugeTests</c> pass alone and fail under the full
 /// concurrent suite. Each test also removes its own connections in a <c>finally</c> so it
-/// leaves the shared registry exactly as it found it.
+/// leaves the shared registry exactly as it found it. The class is also in the
+/// non-parallel <c>DataConnectionGauge</c> collection (Gitea #99), because the deltas can still be
+/// corrupted by another class mutating the registry between baseline and assertion.
 /// </remarks>
+[Collection(DataConnectionGaugeCollection.Name)]
 public sealed class DataConnectionGaugeTests
 {
     private const string ConnectedGauge = "scadabridge.site.data_connections.connected";

tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Observability/ExtendedInstrumentTests.cs

@@ -9,6 +9,7 @@ namespace ZB.MOM.WW.ScadaBridge.Commons.Tests.Observability;
 /// <see cref="MeterListener"/> filtered to the emitting (test) thread, because the meter is
 /// process-global and other tests emit on it concurrently.
 /// </summary>
+[Collection(DataConnectionGaugeCollection.Name)]
 public sealed class ExtendedInstrumentTests
 {
     private static List<(long Value, Dictionary<string, object?> Tags)> Capture(string instrument, Action act)