Enable mutex profiling for Cloud Profiler
This enables mutex profiling in cloud profiler.
The fraction is hard-coded to 1%: https://github.com/googleapis/google-cloud-go/blob/master/profiler/mutex.go#L21.
I suspect it was disabled by default for BC reasons: Mutex profiling was introduced in Go 1.8.
These profiles will show up as "contention" in the Cloud Profiler UI.