Potential histogram bucket overcounting

[kriskowal]

Tracking on yarpc/yarpc-go#1882, we’ve disabled a couple flappy tests because due to intermittent over-counting in the latency histogram buckets.

These tests create a single request for the lifetime of the metrics graph. They assert that one request should appear in a latency histogram bucket. It’s usually just one. Occasionally in CI, tests would fail with more than one counted request.