[FLINK-8946] TaskManager stop sending metrics after JobManager failover

[yanghua]

What is the purpose of the change

This pull request fixed taskManager stop sending metrics after JobManager failover

Brief change log

• Removed TaskManagerMetricGroup#close from method disassociateFromJobManager

Verifying this change

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

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)
• The S3 file system connector: (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)

[yanghua]

cc @zentol I recreate a new PR base on your suggestion #5869 , the travis build failed because of flink-statebackend-rocksdb

[yanghua]

cc @zentol have a look at this?

[yanghua]

cc @zentol @tillrohrmann

[zentol]

merging.