One of go-kit dependencies is not goroutine safe

[gaplyk]

Got a panic in one of dependencies when 2 routines trying to write metrics:
https://github.com/VividCortex/gohistogram/blob/master/numerichistogram.go#L124

As for author comment it is not goroutine safe:
VividCortex/gohistogram#21

One of the options is to add mutex to generic histogram here:
https://github.com/go-kit/kit/blob/master/metrics/generic/generic.go#L121