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.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();
         }
     }