Pytest should clear collected metrics after each test runs

[brettimus]

Right now it's possible for the metrics generated by one test to be picked up by another test.

Each test should somehow "clear" the collected metrics, so that their actions do not pollute the result of (e.g.) generate_latest

[brettimus]

I think I hit this issue when working on #54 — some tests passed that should have failed because they were comparing against metrics generated by other tests