[FLINK-38821] [metrics] Add TaskManager creation/deletion metrics

[moonyoungCHAE]

What is the purpose of the change

I added the metrics for TaskManager creation and deletion events count.

Although a metric tracking the current number of TaskManagers already exists, it does not capture short-lived TaskManagers that are created and removed within a brief time window. To improve observability in such scenarios, this change adds cumulative (total) metrics for TaskManager creations and deletions, enabling better tracking and analysis of transient lifecycle events.

Brief change log

Added cumulative (total) metrics for TaskManager creation events
Added cumulative (total) metrics for TaskManager deletion events

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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 / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• b2080e3 Azure: FAILURE

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