Document metrics and healthcheck listener configuration.

[jpeach]

What steps did you take and what happened:

In #2407 , extra CLI flags were added to allow the metrics and healthcheck services to listen on different address and ports. We need to document these new flags and describe the circumstances under which they should be used.

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[github-actions[bot]]

The Contour project currently lacks enough contributors to adequately respond to all Issues.

This bot triages Issues according to the following rules:

• After 60d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, the Issue is closed

You can:

• Mark this Issue as fresh by commenting
• Close this Issue
• Offer to help out with triage

Please send feedback to the #contour channel in the Kubernetes Slack