ISSUE-500: Make Topic Level Prometheus Metrics Optional

[sijie]

Original Issue: apache#500

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Is your feature request related to a problem? Please describe.
In 0.4.0, apache#410 introduced topic level prometheus metrics. When consuming from or publishing to many topics, this creates many metrics and can cause problems for applications and/or prometheus instances. In my case, I saw up to 900k metrics produced by a single client, which caused memory problems in my application and temporarily brought down my prometheus server.

Describe the solution you'd like
I would like to be able to enable or disable the topic level metrics. I think these topic level metrics should be disabled by default, since that was the previous behavior.

Describe alternatives you've considered
I looked into the code introduced in 0.4.0, and I don't see any ways to prevent these metrics from being created currently.

Additional context
I like the metrics I get out of the client in 0.3.0. I think it's great to have them out of the box.

Also, the cardinality of metrics is preventing me from upgrading my go client from 0.3.0 to 0.4.0.