Add opt-in target host and port observation tags

[henriquejsza]

Summary

• add an opt-in TargetHostAndPortFeignObservationConvention
• emit net.peer.host and an explicitly configured net.peer.port
• preserve the existing default low-cardinality key set
• safely omit target tags for URL-less or invalid targets
• document activation, port semantics, and cardinality considerations

supportsContext is declared explicitly with the existing semantics so JApiCmp can validate the new public convention.

Testing

• ./mvnw -Pdev -Dtoolchain.skip=true -pl micrometer -am verify

Fixes #2630

[velo]

Nice addition — subclassing DefaultFeignObservationConvention and wiring in through MicrometerObservationCapability's existing customConvention extension point is exactly the right seam for this. A couple of things before merging:

1. supportsContext is overridden with a body identical to the inherited default (context instanceof FeignContext). The PR description says it's there so japicmp can validate the new convention, but that doesn't apply — this is a brand-new class with no prior release baseline, and the sibling DefaultFeignObservationConvention it extends doesn't override this either. Please drop the override; inheritance already does the right thing.

2. targetUri() catches URISyntaxException | UnsupportedOperationException around feignTarget().url(), but RequestTemplate.feignTarget() can return null for any RequestTemplate not routed through RequestTemplateFactoryResolver (e.g. a hand-built RequestTemplate from a custom Client or test double). That path would NPE uncaught, which defeats the class's stated guarantee that "a missing or invalid url never breaks the observation." Please null-check feignTarget() explicitly (or widen the catch) so that guarantee actually holds.

Smaller, non-blocking:

• No test exercises the URISyntaxException branch in targetUri() — worth a case for parity with the other catch arm.
• README typo: "urless" → "url-less".

Happy to merge once 1 and 2 are addressed.