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.