[feat][broker] PIP-264: Add broker web executor metrics

[dragosvictor]

PIP-264

Motivation

Adds metrics for the web service executor thread pool, as currently described here. Reference documentation for the new metrics can be found in the respective PR: apache/pulsar-site#907.

Modifications

Expose the respective OpenTelemetry metrics in new class org.apache.pulsar.broker.web.WebExecutorThreadPoolStats#WebExecutorThreadPoolStats.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Extended test org.apache.pulsar.broker.web.WebServiceTest#testWebExecutorMetrics to validate the OpenTelemetry metrics

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

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics: As describe above
• Anything that affects deployment

Documentation

• doc
• doc-required: [feat][broker] PIP-264: Add web executor metrics reference pulsar-site#907
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: dragosvictor#27

[lhotari]

LGTM