Export to StackDriver Monitoring

[jparsana]

1. Convert OpenCensus Stats/Metrics data model to Stackdriver data model.
2. Set correct domain name prefix for uploaded Stackdriver metrics.
3. Auto-detect and populate MonitoredResource and associate it with metrics (tracking issue Ruby).
4. Add one additional task label for metrics to avoid potential race condition on the same machine. See what-is-opencensus_task-metric-label.
5. E2E tests to verify exporting to actual Stackdriver backend, especially when there are multiple exporters running simultaneously.

[dazuma]

This task should be implemented in the https://github.com/census-ecosystem/opencensus-ruby-exporter-stackdriver library.