profiler: add WithTelemetryDisabled for OpenCensus users (#1543)
profiler: add WithTelemetryDisabled for OpenCensus users (#1543)

In Cloud Trace and Cloud Monitoring, we officially recommend to use OpenCensus as the instrumentation framework.

* https://cloud.google.com/trace/docs/setup/go
* https://cloud.google.com/monitoring/custom-metrics/open-census

However, when it comes to use them with Cloud Profiler agent, Trace UI is polluted with a lot of Spans generated from the Profiler agent, because it sends profile data to the backend via gRPC and the gRPC dialer comes with OpenCensus's ClientHandler.

* https://github.com/googleapis/google-api-go-client/blob/00ea0dae70a28d5e659ad288fdee5f7f5ab56a96/transport/grpc/dial.go#L173

In order to avoid the pollution on Cloud Trace UI and to reduce the billing cost to Cloud Trace, we should put some notes in the example for OpenCensus users to disable unnecessary telemetries from the Profiler agent. 
GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples