[FLINK-7368][metrics] Backport synchronization fix for MetricStore

[pnowojski]

What is the purpose of the change

This backports adding missing synchronization on MetricStore

Verifying this change

This change is already covered by existing tests, such as MetricFetcherTest.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)

Documentation

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

[zentol]

There is another occurrence in TaskManagersHandler (

flink/flink-runtime-web/src/main/java/org/apache/flink/runtime/webmonitor/handlers/TaskManagersHandler.java

Line 112 in 80c23de

MetricStore.TaskManagerMetricStore metrics = fetcher.getMetricStore().getTaskManagerMetricStore(instance.getId().toString());

)

[pnowojski]

@zentol indeed, thanks for pointing out. There was one more place MutableIOMetrics.java.

[zentol]

merging.

[zentol]

@pnowojski I've merged the PR, could you close it?

[pnowojski]

Yes sure, thanks for merging