[feat][misc] PIP-264: Add OpenTelemetry messaging rate limit metrics

[dragosvictor]

PIP-264

Motivation

Adds messaging rate limit metrics, currently exposed in classes InflightReadsLimiter and ServerCnxThrottleTracker, to the OpenTelemetry pipeline.

Modifications

• Added metrics, as described by [feat][doc] PIP-264: Add OpenTelemetry messaging rate limit metrics reference pulsar-site#943, to the OpenTelemetry pipeline.
• The metrics differ slightly from the existing ones:
  • Connection throttling metrics are cumulative totals, instead of the instantaneous value in the existing Prometheus metric pulsar_broker_throttled_connections. This gives better visibility into the frequency of rate limiting operations.
  • Connection throttling metrics are exposed for pause/resume operations too.
  • Managed ledger inflight reads metrics expose both the hard-wired limit and current usage, for increased visibility.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Updated tests InflightReadsLimiterTest#testDisabled and InflightReadsLimiterTest#testBasicAcquireRelease to cover the changes to the InflightReadsLimiter metrics
• Updated tests in MessagePublishBufferThrottleTest to verify connection pause/unpause metrics
• Added test MessagePublishBufferThrottleTest#testConnectionThrottled to verify connection throttle/unthrottle 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 described in [feat][doc] PIP-264: Add OpenTelemetry messaging rate limit metrics reference pulsar-site#943
• Anything that affects deployment

Documentation

• doc
• doc-required [feat][doc] PIP-264: Add OpenTelemetry messaging rate limit metrics reference pulsar-site#943
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: dragosvictor#41