Summary:
Refactor the startup performance API so that we are not using a vector to store only a few startup metrics values. This makes the metrics more strictly typed and concise.
Changelog: [Internal]
Reviewed By: mdvacca
Differential Revision: D43860049
fbshipit-source-id: 2c2102de2b64e2c5cda509ac26f0d1d072287083