[fix][broker] Fix configuration exposeSubscriptionBacklogSizeInPrometheus not working

[Shawyeok]

Motivation

Broker configuration exposeSubscriptionBacklogSizeInPrometheus was introduced in #9302, but it is true, broker just calculate the size of backlog and didn't expose it.

Modifications

• if and only if exposeSubscriptionBacklogSizeInPrometheus=true, expose the metric as pulsar_subscription_back_log_size

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change added tests and can be verified as follows:

• if exposeSubscriptionBacklogSizeInPrometheus=false, it shouldn't expose the metric
• if exposeSubscriptionBacklogSizeInPrometheus=true, it should expose the metric

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

If the box was checked, please highlight the changes

• 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
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: Shawyeok#6

[Shawyeok]

/pulsarbot rerun-failure-checks

[Shawyeok]

/pulsarbot rerun-failure-checks

[Shawyeok]

PTAL @codelipenghui

[gaoran10]

Nice work!

[codelipenghui]

@Shawyeok A PIP is required If the change will introduce a new indicator to the metrics endpoint. Please see https://github.com/apache/pulsar/blob/master/wiki/proposals/PIP.md and the discussion under the mailing list https://lists.apache.org/thread/bmwtk6zcyt4ft4t95jmz1bnsxbysjw8m

I support the motivation.

[codelipenghui]

PIP is required.

[Shawyeok]

@Shawyeok A PIP is required If the change will introduce a new indicator to the metrics endpoint. Please see https://github.com/apache/pulsar/blob/master/wiki/proposals/PIP.md and the discussion under the mailing list https://lists.apache.org/thread/bmwtk6zcyt4ft4t95jmz1bnsxbysjw8m

I support the motivation.

Ok, I'll draft a PIP in the weekend.

[github-actions]

The pr had no activity for 30 days, mark with Stale label.