Switching underlying metrics provider from rcrowley/go-metrics to go-kit/kit/metrics

[jprobinson]

• config.Server.GraphiteHost is now a deprecated config.
• New config.Metrics, config.Config.Metrics and config.Server.Metrics handle all metrics configuration.
• Now able to integrate with statsd, dogstatsd, prometheus, expvar, graphite or nothing at all.
• Changes to emitted server route metric names:
  • Counter names no longer have a .count suffix
  • Histogram quantile names change from suffixes like 50-percentile to _50p

[coveralls]

Coverage decreased (-0.6%) to 44.966% when pulling 31fdbe0 on kit-metrics into 5761566 on master.

[coveralls]

Coverage increased (+0.4%) to 45.966% when pulling abcca4e on kit-metrics into 5761566 on master.