Consider using sync/atomic for Go SDK metrics.

Changing a portion of the metrics code to use the atomic counters might yield a performance improvement and the opportunity to remove a lock or two.

Care needs to be taken though: https://stackoverflow.com/questions/47445344/is-there-a-difference-in-go-between-a-counter-using-atomic-operations-and-one-us

The outcome of this task is a benchmark demonstrating the benefit (or detriment) in a quasi-real situation for the Go SDK, and if warranted switching metrics where possible, to use atomics.