Add TLS Support for Metrics endpoint

Description

As part of the effort to enable TLS encryption between GitLab services - the metrics/debug endpoint for the GitLab Runner should be able to export metrics over https.

As part of working out the configuration needed for our in-chart Prometheus scrape configurations to support metrics exporter endpoints configured for TLS (see gitlab-org/charts/gitlab#3335+ ) - it looks like there's not currently configuration support to have the Runner metrics/debug endpoint make use of TLS.

Proposal

Implement configuration to allow for a certificate/key pair to be provided along with --listen-address and used to expose the metrics/debug using TLS.

Examples/support/notes in other projects:

• Pages MR: Add TLS for metrics (gitlab-pages!772 - merged)
• Registry Issue: Add support of TLS on `http.debug` (container-registry#729 - closed)
• GitLab Shell comment: gitlab-shell#549 (comment 936480946)

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@DarrenEastman @erushton Just an FYI that at some point for a full end-to-end TLS configuration, the Runner metrics endpoint will need TLS support.

@jayo Moving this issue to the backlog.

mentioned in merge request gitlab-org/charts/gitlab!2671 (merged)

mentioned in issue gitlab-org/cluster-integration/gitlab-agent#288 (closed)

Setting label(s) devopsverify sectionops based on grouprunner.

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mentioned in issue gitlab-org/quality/triage-reports#8499 (closed)

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mentioned in issue gitlab-org/quality/triage-reports#8564 (closed)

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mentioned in issue #29507 (closed)

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mentioned in issue #29287 (closed)

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@nicolewilliams thanks for adding the Seeking community contributions label!

This issue's description does not seem to have a section for "Implementation Guide". Please consider adding one, because it makes a big difference for contributors. This section can be brief but must have clear technical guidance, like:

• Hints on lines of code which may need changing
• Hints on similar code/patterns that can be leveraged
• Suggestions for test coverage
• Ideas for breaking up the merge requests into iterative chunks
• Links to documentation (within GitLab or external) about implementation or testing guidelines, especially when working with third-party libraries

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