separate out raintank's metric package out of grafana into standalone

[Dieterbe]

pkg/metric

so that it's reusable to other apps and we don't need a fork of grafana.

[woodsaj]

@Dieterbe as you have been working on updating the metrics lib, can we address this too.

[Dieterbe]

the reason of pkg/metric's existence is it can neatly switch over between regular statsd and dogstatsd tags. but now that i've found a statsd client that looks like the highest performance - https://github.com/alexcesaro/statsd - and it also offers optional datadog and influx tags. there's no real need for this package at all anymore, and we can use this statsd client directly. ( and anyway we don't use DD anymore and presumably other people who would run litmus won't either. ) I will update everything in raintank-metric to use the new client directly, and then raintank/grafana and then we can just remove it this package.

[Dieterbe]

actually, an abstraction still makes sense for these reasons:

• it's nice to have variables representing gauges, timers etc for robustness (esp when used in multiple places), instead of relying on string arguments and having a central overview
• it'll be nice if we also decide to abstract another library such as go-metrics
• it saves me a bunch of time if i can just relocate the lib :) so that's what i'll do for now.