* Refactor existing metrics support into MetricHelper

This completes the refactor done at https://github.com/badges/shields/pull/3662#issuecomment-509011229 in anticipation of adding more metrics support, such as response size of an upstream service, or response time.

* Clean up

* Renames

* Add response time metrics

This adds around 30 new metrics to cover response times at a fairly granular level. We may be able to shrink the number of buckets with time, though I think using 30 metrics is probably okay given that I think may become our most important metric.

* Fix