[FLINK-37558] Check right variable in addSpan

[chanhyeong]

What is the purpose of the change

Looking at the null check cases for 'reporters' in MetricRegistryImpl, the null check in addSpan should be changed to the null check for 'traceReporters' rather than 'reporters', because notifyTraceReportersOfAddedSpan method uses 'traceReporters', not 'reporters'.

The null check may be meaningless because all reporters values are initialized to empty ArrayList from the constructor, but I'd like to make the change for the correct check.

Brief change log

Replace the null check variable 'reporters' to 'traceReporters' in MetricRegistryImpl#addSpan

Verifying this change

This change is already covered by existing tests, MetricRegistryImplTest

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): yes (public method)
• 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

[flinkbot]

CI report:

• b313cc4 Azure: SUCCESS

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[davidradl]

Looks good - but can we unit test this change?

[chanhyeong]

Thank you for comment!
I also checked MetricRegistryImplTest before push this commit, but there was no tests for null cases.
As mentioned in pr summary, I thought this can't be reproducible because all reporters values are initialized to empty ArrayList.
If you let me know a good way to reproduce cases, I will apply that.

However, this change is no longer valid because #26316, which is expected to in deployed in version 2.1, was merged.
If you don't mind for applying this change, could I change the base branch to release-2.0?