Stackdriver exporter: use project ID from GCP metadata when possible

[draffensperger]

Currently the Stackdriver trace/metrics explorer requires a project configuration field. However, in cases when the customer is running their agent/collector on GCP, it would be nice to default the project based on the GCE metadata.

That would enable easier deployment scripts for Kubernetes because they would not need to have the project parameter substituted with the customer project, but could just auto-determine that based on what GCP project the agent is running in.