Opportunity: OpenTelemetry trace-context propagation (SEP-414)

[jongalloway]

Opportunity: OpenTelemetry trace-context propagation (SEP-414)

Part of #481.

SDK 2.0 propagates W3C Trace Context (traceparent/tracestate) across JSON-RPC messages built-in, using ActivitySource/Meter "Experimental.ModelContextProtocol". This complements the existing telemetry stack (DotNetMcp/Telemetry/*).

Tasks

• Verify incoming trace context is extracted and parented to server-side activities once on 2.0.
• Ensure the existing telemetry/metrics don't conflict with the SDK's ActivitySource/Meter names.
• Document distributed-tracing behavior in doc/telemetry.md.

Acceptance

• Trace context flows end-to-end and is reflected in the project's telemetry docs.