Add Prometheus as domain expertise

Why is this change being made?

I have been looking to find reviewers for MRs recently who can review PromQL and other Prometheus related changes, especially since it is now becoming more important to the product itself as well with Self-Monitoring and Usage Pings.

It would be great to have it listed as domain_expertise so that folks with more experience in that area can add themselves, and others can find them more easily for questions and reviews.

Once added, people can add this option in data/team.yml and will appear on https://about.gitlab.com/handbook/engineering/projects/ as having Prometheus expertise.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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