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/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)