[client] Add MetricsRegisterer to ClientOptions

[pragkent]

Add ClientOptions.MetricsRegisterer for customizing prometheus
metrics registerer.

Motivation

Currently, client always use prometheus.DefaultRegisterer to register metrics, but the application might already have a registerer for metrics registering and exporting.

Modifications

Add ClientOptions.MetricsRegisterer to allow user to customize the metrics registerer. They can use the existed registerer in their applications.

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.

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? (yes)
• If yes, how is the feature documented? (GoDocs)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[wolfstudy]

Nice work, LGTM +1