Add baggage tag fields to spans with OpenTelemetry and document gRPC baggage propagation

[o-shevchenko]

This PR replaces the original proposal (auto-configured gRPC baggage interceptors). While porting it from spring-projects/spring-grpc#369 I verified the behaviour end-to-end on main and on 4.1.1, and it turned out that Spring Boot already propagates baggage over gRPC out of the box — see the details below. What remained is a tagging gap that is not gRPC-specific, plus missing documentation. Related to #49832.

What already works (no code change needed)

With spring.grpc.{server,client}.observation.enabled (default) and Micrometer Tracing configured, ObservationGrpcServerInterceptor / ObservationGrpcClientInterceptor hand the gRPC metadata to PropagatingReceiver/SenderTracingObservationHandler, which run the same Propagator chain as HTTP. A probe with the in-process transport, the OTel SDK and W3C propagation showed, for both main and 4.1.1:

• server: the W3C baggage header and the individual management.tracing.baggage.remote-fields keys are extracted and visible via Tracer#getBaggage inside unary and server-streaming methods, no leaks between calls
• client: current baggage is written as baggage header and as individual remote-field keys, plus traceparent

The only prerequisite is micrometer-core on the classpath (the interceptors live there); without it, gRPC gets no observation and therefore no propagation, silently.

What did not work: tag fields

management.tracing.baggage.tag-fields were not applied to the span of an incoming request unless the application itself called tracer.getBaggage(...). That is micrometer-metrics/tracing#933 — the OTel bridge builds the span's parent Context from Context.current() and drops the extracted baggage, so SpanProcessor#onStart never sees it. I've proposed a fix for the bridge in micrometer-metrics/tracing#1548.

Changes in this PR

• OpenTelemetryPropagationConfigurations: register Micrometer's BaggageTaggingSpanProcessor when baggage is enabled and at least one tag field is configured (@ConditionalOnMissingBean, so a custom bean backs off). Today this tags spans that start while baggage is current (client and internal spans); together with the bridge fix it also covers server spans of incoming requests.
• gRPC reference docs: new server "Observability" section and an extended client one describing that trace context and baggage (W3C header + remote fields) are propagated by the auto-configured observation interceptors, the micrometer-core prerequisite, and the spring.grpc.*.observation.enabled switches.
• Tracing reference docs: mention tag-fields and gRPC in the Baggage section.
• Tests for the new bean (conditions, back-off, and a functional check that a span started with baggage in scope is tagged).