[FLINK-35405][runtime] Add metrics for AsyncExecutionController

[fredia]

What is the purpose of the change

Add metrics for AsyncExecutionController.

Brief change log

• Add metrics for AsyncExecutionController.
• Add AEC section in metrics.md

Verifying this change

This change added tests and can be verified as follows:

• AsyncExecutionControllerTest#testBasicRun()

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)

[Zakelly]

Thanks for the PR!

[flinkbot]

CI report:

• 9fcb0b0 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[Zakelly]

LGTM