Don't add resources to Multimeter when there's no multimeter running

Summary: Fixes T8058. Don't add a Multimeter event if we aren't running multimeter (e.g., in unit tests).

Test Plan: Will arc unit --everything.

Reviewers: joshuaspence, btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8058

Differential Revision: https://secure.phabricator.com/D12699