Add TLS for metrics

What does this MR do?

Add support for TLS for metrics as per discussion in #705 (comment 959507571).

Allow users to specify the certificate/private-key to use for TLS communication related to metrics.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines