Pretty good. Some opinions that fall under "be consistent within our code base" (which is why Uber should have a style guide at all), so all good.
Just one comment though:
> Embedding sync.Mutex
Never do this on an exported type. If you embed sync.Mutex that makes "Lock" and "Unlock" part of your exported interface. Now the caller of your API doesn't know if they are supposed to call Lock/Unlock, or if this is a pure internal implementation detail.
your `type Stats` "isn't" a mutex. It has a mutex.
Agree, but I can't recall ever seeing a package in the wild that labels itself as "concurrency-safe" and yet requires the caller to explicitly call Lock!
HashMap is probably a good example