Enable mutex profiling for Cloud Profiler

Igorrequested to merge
mutex-profiling into master

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.

cc @oswaldo @jacobvosmaer-gitlab

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