Add utils to automatically detect monitored resources.

[songy23]

Add utils to automatically detect monitored resources based the environment that the application is running in, according to OpenCensus MonitoredResource specs. This util should be independent of specific exporters, since it's supposed to be share between Stackdriver Stats exporter and Stackdriver Trace exporter.

Counterpart in Java: https://github.com/census-instrumentation/opencensus-java/tree/master/contrib/monitored_resource_util.

[dazuma]

There is an existing gem google-cloud-env that talks to the GCP metadata service. Please use that library on the GCP side.