Initial CloudWatch metric reporting support

Summary: Ref T12857. Ref T13076. Initial infrastrucutre for creating and submitting CloudWatch metrics to AWS.

Test Plan: Submitted some metrics, saw them show up.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T13076, T12857

Differential Revision: https://secure.phabricator.com/D19363