Use isNoop() instead of Observation.NOOP

[jonatan-ivanov]

TL;DR: this is a polishing PR as long as micrometer-metrics/micrometer#6700 is not merged (1.16.0). If that will happen, things can break without this change.

Details:
The Micrometer team is considering some improvements in the Observation API which involves changing the behavior of Observation.NOOP: micrometer-metrics/micrometer#6700
Observation.NOOP is not truly no-op right now, it does context propagation, we are trying to make it truly no-op in the PR above. The non-truly no-op version will still be used in certain scenarios (see NoopButScopeHandlingObservation). observation.isNoop() should be used for checking if an Observation is no-op or not instead of Observation.NOOP.

[jonatan-ivanov]

Not sure what is going on, the build is failing with this (locally too):

Execution failed for task ':spring-integration-kafka:compileTestJava'.
> Compilation failed; see the compiler output below.
  /home/runner/work/spring-integration/spring-integration/spring-integration-kafka/src/test/java/org/springframework/integration/kafka/inbound/MessageSourceTests.java:338: error: reference to close is ambiguous
  		inOrder.verify(consumer).close(any());
  		                        ^
    both method close(Duration) in Consumer and method close(CloseOptions) in Consumer match

which seems unrelated to the change but main seems ok.