Remove observability/metrics Service port from gitlab-kas

What does this MR do?

Removes the port that we don't need to expose on the Service. See gitlab-org/cluster-integration/gitlab-agent!160 (comment 447735935). This is a follow up for !1646 (merged).

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened