micrometer: ObservationThreadLocalAccessor.restore() generating errors in tests

Describe the bug When upgrading from spring-boot 3.0.4 (micrometer 1.10.4) to spring-boot 3.0.8 or 3.1.1 (micrometer 1.11.1), some of my tests are now failing with a message comparing a NoopObservation with null :

Observation <io.micrometer.observation.NoopObservation@390cd91e> to which we’re restoring is not the same as the one set as this scope’s parent observation <null> . Most likely a manually created Observation has a scope opened that was never closed. This may lead to thread polluting and memory leaks

It seems related to #3831 which introduced NullObservation and the verification in ObservationThreadLocalAccessor.restore().

Environment

• Micrometer version 1.11.1
• OS: Ubuntu
• Java version: openjdk version “17.0.1” 2021-10-19

To Reproduce

I’m trying to extract a test case from my own code to show how to reproduce it. I will post it here once I’m able to reproduce it on a sample application.

For the moment, it seems related to observations created by Mono.tap(Micrometer.observation(registry)).

Exemple of test that fails with the new version :

  @Test
  @DisplayName("Save when the bucket does not exist. Must terminate with errors.")
  void saveWhenBucketDoesNotExist() throws IOException {
    // GIVEN
    final Resource fileResource = ClasspathResourceLoader.fromFile("input/ID-card.png");

    // WHEN
    final Mono<FileReference> responseFileReferenceMono =
        s3Repository.save(fileResource.getFile().toPath());

    // THEN
    StepVerifier.create(responseFileReferenceMono)
        .expectError(NoSuchBucketException.class)
        .verify();
  }

Code under test :

  public Mono<FileReference> save(final Path path) {
    return save(AsyncRequestBody.fromFile(path), path.toFile().length());
  }

  private Mono<FileReference> save(
      final AsyncRequestBody asyncRequestBody,
      final long length) {
    return Mono.fromFuture(() -> putObject(asyncRequestBody, fileReference, length))
        .name("s3 put-object")
        .doOnError(NoSuchBucketException.class, e -> LOGGER.error("Unknown bucket"))
        .tap(Micrometer.observation(observationService.registry()))
        .thenReturn(fileReference);
  }

  private CompletableFuture<PutObjectResponse> putObject(
      final AsyncRequestBody asyncRequestBody,
      final long length) {
    final PutObjectRequest request =
        PutObjectRequest.builder()
            .bucket("test")
            .key("myKey")
            .contentLength(length)
            .contentType(MediaType.APPLICATION_OCTET_STREAM.getType())
            .build();
    return s3client.putObject(request, asyncRequestBody);
  }

Expected behavior Test should not fail when testing an error happening inside a reactive stream which is observed.

Additional context

• #3831