Allow adding span links through Observation

[MakoriSM]

Currently, Micrometer’s Observation API provides a convenient abstraction over metrics and tracing, but it doesn't expose any way to create links between spans.

This makes it difficult to represent my “fan-out” scenario where:

Multiple caller threads trigger the same underlying work (e.g. multiple threads hitting a cache miss but only one performs the computation).

I want each caller’s Observation (e.g. cache.get) to be related to the single execution span (e.g. cache.output) through a link, rather than a strict parent/child relationship.

I have attempted to achieve this by dropping down to the tracing layer (e.g. unwrapping OtelSpan via TracingObservationHandler.TracingContext and calling io.opentelemetry.api.trace.Span.addLink):

TracingObservationHandler.TracingContext tc =
    observationA.getContext().get( TracingObservationHandler.TracingContext.class );
Span span = tc.getSpan();
io.opentelemetry.api.trace.Span otelSpan = ( ( OtelSpan ) span ).delegate;
otelSpan.addLink( OtelTraceContext.toOtelSpanContext( ( TraceContext ) observationB ) );

But this doesn't work as OtelSpan is protected and there are no other clear ways to turn a micrometer span into an opentelemetry span.

Expected Behavior

It should be possible to do something like:

Observation fetchObs = Observation.start("cache.output", registry);
Observation current = registry.getCurrentObservation();

// Proposed API
current.addLink(fetchObs);

which under the hood would delegate to the tracing implementation’s span builder (e.g. OTel addLink or Brave equivalent).

Actual Behavior

Observation has no concept of links.

[marcingrzejszczak]

Yeah this one is tricky. I guess it doesn't make a lot of sense to copy the tracing api into observation. Otoh linking observations might make sense - maybe not for the metrics signal but any other one. Maybe an optional description on the link would be useful?