[Adiakys]

Summary

AnyMediator now emits OpenTelemetry-compatible spans and a duration histogram through .NET's own ActivitySource and Meter: no new dependency, nothing recorded until something subscribes.

builder.Services.AddOpenTelemetry()
    .WithTracing(t => t.AddSource(AnyMediatorDiagnostics.ActivitySourceName))
    .WithMetrics(m => m.AddMeter(AnyMediatorDiagnostics.MeterName));

• New public class AnyMediatorDiagnostics with ActivitySourceName and MeterName (minor version bump at release: 2.2.0).
• Spans: send {Request}, create_stream {Request} (open until the stream is read, fails or is abandoned), publish {Notification} with one handle {Notification} child per handler. Attributes: anymediator.operation, anymediator.message.type, anymediator.handler.type. Failures set Error status, error.type and an exception event per the OpenTelemetry conventions.
• Metric: anymediator.operation.duration histogram (seconds) by operation, message type and error.type.
• Cost without listeners: two flag checks; traced paths live in separate methods so no closure is allocated. Send unchanged at 93 ns / 96 B; Publish 196 → 207 ns, same allocations.
• Docs: new "Tracing and metrics" page (setup, span/metric reference, propagation through a notification sink), README section, llms.txt entry.

Tests

• 128 tests pass (4 new in TelemetryTests, using ActivityListener/MeterListener: send span and duration, failure recording, publish span with per-handler children, stream span lifetime). Ran the suite 4 times to check stability, since listeners are process-wide.
• Docs site builds.

🤖 Generated with Claude Code

[Adiakys]

Added two commits after measuring the cost with a listener active (new TracingBenchmarks, listener recording every span and measurement):

	Not listening	Listening, before	Listening, now
Send (1 span)	87 ns, 96 B	652 ns, 880 B	564 ns, 704 B
Publish, 3 handlers (4 spans)	307 ns, 632 B	2,950 ns, 3,936 B	2,310 ns, 2,872 B

Span names and attributes are now built once per message type, traced operations use a state tuple with static lambdas (no closures), and the extra async layer is gone. The tracing guide's "Cost" section shows these numbers. Still 128 tests passing.