feat(telemetry): export the site script compile-cache eviction counter as a Meter (ScadaBridge#68)

SiteScriptCompileCache now records each eviction sweep on scadabridge.site.script_compile_cache.evictions
(ScadaBridgeTelemetry). It rides the existing ZB.MOM.WW.ScadaBridge meter, which is already in
SiteServiceRegistration.ObservedMeters, so no new allowlist entry is needed; a Host test pins that
the meter stays allowlisted.

src/ZB.MOM.WW.ScadaBridge.Commons/Observability/ScadaBridgeTelemetry.cs

@@ -76,6 +76,17 @@ public static class ScadaBridgeTelemetry
         Meter.CreateCounter<long>("scadabridge.site.alarm_cache.buffer_dropped", unit: "1",
             description: "Live deltas evicted from a live-alarm aggregator's bounded pre-seed buffer (drop-oldest).");
 
+    /// <summary>
+    /// Incremented by the number of entries the site script compile cache evicts when it crosses its
+    /// bound (ScadaBridge#68). Eviction does not unload the Roslyn assembly, so each recompile of an
+    /// evicted body loads another permanent copy: a sustained climb means compile-cache thrash and
+    /// monotonic working-set growth - raise <c>ScadaBridge:SiteRuntime:ScriptCompileCacheMaxEntries</c>.
+    /// Alert on this instead of the rate-limited Warning log.
+    /// </summary>
+    private static readonly Counter<long> _scriptCompileCacheEvictions =
+        Meter.CreateCounter<long>("scadabridge.site.script_compile_cache.evictions", unit: "1",
+            description: "Entries evicted from the site script compile cache at its bound (each recompile of an evicted body loads another permanent Roslyn assembly).");
+
     /// <summary>
     /// Incremented for each debug event evicted from a central debug session's bounded
     /// pre-snapshot buffer (drop-oldest). The Debug View is lossy-under-backpressure
@@ -370,6 +381,10 @@ public static class ScadaBridgeTelemetry
     /// <param name="count">Number of deltas evicted.</param>
     public static void RecordLiveAlarmBufferDrop(long count = 1) => _liveAlarmBufferDrops.Add(count);
 
+    /// <summary>Records entries evicted from the site script compile cache.</summary>
+    /// <param name="count">Number of entries evicted by one sweep.</param>
+    public static void RecordScriptCompileCacheEvictions(long count) => _scriptCompileCacheEvictions.Add(count);
+
     /// <summary>Records debug events evicted from a debug session's bounded pre-snapshot buffer.</summary>
     /// <param name="count">Number of events evicted.</param>
     public static void RecordDebugPreSnapshotDrop(long count = 1) => _debugPreSnapshotDrops.Add(count);

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/SiteScriptCompileCache.cs

@@ -2,6 +2,7 @@ using System.Collections.Concurrent;
 using System.Security.Cryptography;
 using System.Text;
 using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.ScadaBridge.Commons.Observability;
 
 namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;
 
@@ -105,6 +106,8 @@ internal sealed class SiteScriptCompileCache
     /// <c>Script&lt;object?&gt;</c>; the assembly Roslyn loaded for it stays in the default load context
     /// for the process lifetime, and a later miss for the same body loads a second copy. A rising value
     /// therefore means monotonic working-set growth — raise <c>ScriptCompileCacheMaxEntries</c>.
+    /// Also exported as the <c>scadabridge.site.script_compile_cache.evictions</c> counter
+    /// (<see cref="ScadaBridgeTelemetry.RecordScriptCompileCacheEvictions"/>) for Prometheus alerting.
     /// </summary>
     public long Evictions => Interlocked.Read(ref _evictions);
 
@@ -183,6 +186,8 @@ internal sealed class SiteScriptCompileCache
                 _cache.TryRemove(victim, out _);
 
             Interlocked.Add(ref _evictions, victims.Length);
+            // Exported on the shared ScadaBridge meter (already in SiteServiceRegistration.ObservedMeters).
+            ScadaBridgeTelemetry.RecordScriptCompileCacheEvictions(victims.Length);
             WarnEvicted();
         }
     }