Create a single Cloud Monitoring client in the built-in metrics #674
Description
Assuming we can get GoogleCloudPlatform/opentelemetry-operations-go#1033 merged, this issue is for creating a single Cloud Monitoring client and configuring the exporter with it.
Right now, each instance the Go Connector dials will get a separate Cloud Monitoring client. The client internally uses gRPC and having multiple clients means extra goroutines are running. Using one client will help the Go Connector be more efficient in its built-in metrics.