Summary: This adds the '/metrics endpoint.

Test Plan: N/A, I'll see if I can add unit tests to it.

Reviewers: vihang, philkuz, #third_party_approvers, michelle

Reviewed By: #third_party_approvers, michelle

Subscribers: vihang

JIRA Issues: PC-1230
Signed-off-by: Zain Asgar <zasgar@pixielabs.ai>

Differential Revision: https://phab.corp.pixielabs.ai/D10124

GitOrigin-RevId: 7c58ceebc20e64ae87c700bfbd1dd6c546771a57