Don't acquire registry lock for runtime metrics.
Don't hold the lock during Each callbacks.
go fmt
Benchmark for testing new EWMA.
Use a lock instead of a goroutine for EWMA.

In this instance, it's way faster.
Style upgrade.
Use a lock instead of a goroutine for Samples.
Style upgrade.
Use a lock instead of a goroutine for Histogram.
Add missing lock around float64 read.
Inline the mutexes into their containers.

This'll be easier on the garbage collector, I think, and is definitely more idiomatic.
Mariano's right: clear the sample inside the lock.
Merge branch 'uncontended'