[FLINK-39617] [runtime] Add batch aggregated subtask metrics endpoints

[Jackeyzhe]

What is the purpose of the change

This pull request adds batch REST endpoints for retrieving aggregated subtask
metrics across multiple job vertices in a single request.

The existing single-vertex endpoint remains unchanged. The new endpoints are
additive GET endpoints and use query parameters instead of request bodies:

• /jobs/:jobid/vertices/subtasks/metrics/names?vertices=<vertexIds>&regex=<regex>
• /jobs/:jobid/vertices/subtasks/metrics/values?vertices=<vertexIds>&get=<metrics>&agg=<aggregations>

The metric name regex parameter uses full Java regex matching, consistent with
the API-shape discussion on FLINK-39617.

Brief change log

• Added batch aggregated subtask metric name and value REST headers, parameters,
  response body, and handlers.
• Moved shared metric-store aggregation logic into
  AggregatedMetricsStoreHelper so both existing and batch handlers can reuse
  the same aggregation behavior.
• Registered the new batch metric handlers in WebMonitorEndpoint.
• Added focused tests for query parameter parsing, response marshalling,
  handler behavior, invalid input handling, and REST endpoint registration.

Verifying this change

This change added tests and can be verified as follows:

• ./mvnw -pl flink-runtime -Dtest=AggregatingSubtasksMetricsHandlerTest,AggregatingSubtasksMetricsBatchHandlerTest,AggregatedSubtaskMetricsBatchParametersTest,AggregatedSubtaskMetricsBatchHeadersTest,AggregatedSubtaskMetricsBatchResponseBodyTest,RuntimeRestAPIVersionTest,WebMonitorEndpointTest test

The command above passed locally with 23 tests, 0 failures, 0 errors, and 0
skipped tests. Checkstyle and Spotless also passed in the same Maven run.

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? yes
• If yes, how is the feature documented? REST API documentation is generated from the REST message headers and parameters added in this change.

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Generated-by: OpenAI Codex (GPT-5)
The changes were reviewed by a human, and I am responsible for the code quality.

[flinkbot]

CI report:

• e693747 Azure: FAILURE

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