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";
+}