Provide an "api" context to Multimeter for API events

Summary: Ref T6930. This gives us a better context for API calls -- "api.call.name" instead of always "web.ConduitAPIController".

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6930

Differential Revision: https://secure.phabricator.com/D12677