Fix missing metrics for topics by registration of existing collector

[pkutilina]

Motivation

It's a bug: when there're several consumers for several topics, builtin pulsar metrics are collected only for first subscribed topic. It happens so because prometheus collector is identified by metric_name + const labels, and as typically topics has predefined metrics set with same const labels, registration throws AlreadyRegisteredError. Ignoring this error leads to metrics left unregistrated (subsequent metrics record observations but they are not collected).

Modifications

Attach existing collector to metrics.
Prometheus lib also suggests handling this type of error this way: https://github.com/prometheus/client_golang/blob/2261d5cda14eb2adc5897b56996248705f9bb840/prometheus/registry.go#L206
https://github.com/prometheus/client_golang/blob/2261d5cda14eb2adc5897b56996248705f9bb840/prometheus/examples_test.go#L602

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The schema: no
• The default values of configurations: no
• The wire protocol: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable
• If a feature is not applicable for documentation, explain why? it's bug, not feature
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation