Expose metrics for dovecot

Summary:
Metrics use the OpenMetrics format.

Ref T1987

Test Plan:

• curl http://172.27.27.3:9900/metrics
• Test in Prometheus with D3388

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1987

Differential Revision: https://devcentral.nasqueron.org/D3387