[broker][monitor] add persistent_subscription_consumer_count for PersistentSubscription

[tjiuming]

Fixes #15032

Motivation

add persistent_subscription_consumer_count for PersistentSubscription

Need to update docs?

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[eolivelli]

Why are we using the Prometheus API ?

Please follow the usual pattern.
If we want to switch to Pure Prometheus API we should discuss this in the community and make a plan

[tjiuming]

Why are we using the Prometheus API ?

Please follow the usual pattern. If we want to switch to Pure Prometheus API we should discuss this in the community and make a plan

There are already so many Prometheus API usages in pulsar, right? Do we need to delete all the usages and discuss if we are allowed to use Prometheus API?

[eolivelli]

In this case the subscription metrics are handled in another way, let's stick to the pattern, like in #15042

https://github.com/apache/pulsar/pull/15042/files#diff-79bde2a6feed80b6b6b4b6950bc7d4ccd93946a8ec0b40cc1bef2e7b3e498f21R271

[tjiuming]

In this case the subscription metrics are handled in another way, let's stick to the pattern, like in #15042

https://github.com/apache/pulsar/pull/15042/files#diff-79bde2a6feed80b6b6b4b6950bc7d4ccd93946a8ec0b40cc1bef2e7b3e498f21R271

I’m confused, how could I know which metrics can be handled by Prometheus and which can't.

[Anonymitaet]

@momo-jun a soft reminder: here is a PR w/ doc-required label, could u pls follow up? Thanks

[codelipenghui]

@tjiuming I'm going to close this PR first since #15042 has merged