Cleaning up TODOs in Metric Classes

This removes fixed or finished TODOs, and ensures they are logically
conistent with work left over. In the future, the TODOs will be removed
alongside the work completion.

Bug: 279611480
Test: Build Test (comment only removal)
Change-Id: I75a6662cdd84f603eb4779e22e8acfdd4b847f7b