Exposing /debug/metrics and /debug/metrics/prometheus was dependent
on --pprof, which also exposes other HTTP APIs. This change makes it possible
to run the metrics server on an independent endpoint without enabling pprof.
* metrics/prometheus: added prometheus http server and metrics collector
* metrics/prometheus: minor cleanups
* metrics/prometheus: named keys instead name in tag
* metrics/prometheus: minor typo cleanups, sorted report
* metrics: expvar support for ResettingTimer
* metrics: use integers for percentiles; remove Overall
* metrics: fix edge-case panic for index-out-of-range
