Helper for reregistering metrics

Summary:
Helpers for if you're registering a metric
that may already exist.

Gogo magic templates!

Test Plan: Expanded unit tests.

Reviewers: psi, arijit, kris, tedo

Reviewed By: tedo

Subscribers: kris, Diffs_emails

Differential Revision: http://phabricator.signalfuse.com/D2877