Global State Race

[sethgrid]

The global state here is a problem and causes data races if multiple implementing servers are spun up at the same time, as can be done in testing:
https://github.com/rcrowley/go-metrics/blob/master/runtime.go#L11

In my code, I start a server like such:

type Server struct {
	config config.Configuration
	metricsPublisher *pfmetrics.Publisher
	metricsRegistry  metrics.Registry
...

In my New method, I set the values like such:

       // srv is a &Server{} and c is config
	srv.metricsRegistry = metrics.NewRegistry()
	srv.metricsPublisher = pfmetrics.NewPublisher(fmt.Sprintf("%s:%s", c.CarbonHost, c.CarbonPort), c.CarbonPrefix, srv.metricsRegistry)

In my tests, I want to run parallel servers. This results in multiple calls to the following in my server's serve method:

	go srv.metricsPublisher.Start()
	defer srv.metricsPublisher.Stop()

Even though the Start and Stop methods are on my srv instance, they affect the global scoped metrics in the library, leading to the following data races.

==================
==================
WARNING: DATA RACE
Read at 0x000001a52df8 by goroutine 61:
  github.com/sendgrid/mta/vendor/github.com/rcrowley/go-metrics.CaptureRuntimeMemStatsOnce()
      /Users/sethammons/workspace/go/src/github.com/sendgrid/mta/vendor/github.com/rcrowley/go-metrics/runtime.go:137 +0xa12
  github.com/sendgrid/mta/vendor/github.com/sendgrid/platformlib/pfmetrics.(*Publisher).Start()
      /Users/sethammons/workspace/go/src/github.com/sendgrid/mta/vendor/github.com/sendgrid/platformlib/pfmetrics/publisher.go:104 +0x469
Previous write at 0x000001a52df8 by goroutine 14:
  github.com/sendgrid/mta/vendor/github.com/rcrowley/go-metrics.CaptureRuntimeMemStatsOnce()
      /Users/sethammons/workspace/go/src/github.com/sendgrid/mta/vendor/github.com/rcrowley/go-metrics/runtime.go:138 +0xa4e
  github.com/sendgrid/mta/vendor/github.com/sendgrid/platformlib/pfmetrics.(*Publisher).Start()
      /Users/sethammons/workspace/go/src/github.com/sendgrid/mta/vendor/github.com/sendgrid/platformlib/pfmetrics/publisher.go:104 +0x469

[eranharel]

I sent a PR for #252 that should fix this:
#253

[sethgrid]

Looked over the PR, seems like it should do it from my perspective! Thanks