[FLINK-37822][tests] Fix flaky test: OpenTelemetryMetricReporterITCase.testReport

[Izeren]

What is the purpose of the change

Address flakiness of the test: OpenTelemetryMetricReporterITCase.testReport

Brief change log

Adding @VisibleForTesting OpenTelemetryMetricReporter#waitForLastReportToComplete method to allow synchronous use of OpenTelemetryMetricReporter#report in tests.

Verifying this change

This change is already covered by existing tests, such as OpenTelemetryMetricReporterITCase.
To ensure it is not flaky, I have run it locally as @RepeatedTest(100) with no issues (previously, fail rate was around 30%).

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• 99db71b Azure: SUCCESS

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