cloudprofiler: Configurable sample rate

[jonhayesverily]

Is your feature request related to a problem? Please describe.
There is no mechanism to configure (reduce) the sample rate of the profiler package, and thus reduce the number of update requests per minute to the Cloud Profiler API. The default GCP quota for Cloud Profiler API update requests is 100 per minute, and the profiler makes 1 update request per profile type per minute. So, 20-25 services in a single GCP project will hit this quota easily with default profile types (faster if you enable mutexes).

Describe the solution you'd like
The profiler.Config type has a positive integer SampleRate field that allows you to specify the number of minutes between samples (default, minimum 1).

Describe alternatives you've considered
Increasing GCP quota for the API