Separate response code class metrics
Separate response code class metrics

Summing over response code totals can produce some unintuitive numbers
due to sharing the 1xx, 2xx, etc. response code classes in the same
metric that exposes the finer-grained numeric response code.

This separates them into `trafficserver_response_classes_total` and
`trafficserver_responses_total`.

gdvalle/trafficserver_exporter