Lameduck alerts, BigQuery Exporter, and Experiment monitoring

This release includes several significant enhancements to the production prometheus configuration.

1. nodes that are in lame-duck mode are excluded from alerts.
2. the bigquery exporter is now available for production dashboards
3. the blackbox exporter supports health checks used by mlab-ns today
4. the gcp-service-discovery side-car service for prometheus, downloads all the new target files including for the inotify exporter.