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: {F391658}
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T6930
Differential Revision: https://secure.phabricator.com/D12677