Managed-prometheus ingestion support in Google Cloud Metrics Starter

Currently metric starter allows ingesting metrics via Stackdriver/Cloud Monitoring api. Recently Google has introduced: https://cloud.google.com/stackdriver/docs/managed-prometheus which can be less expensive:
https://cloud.google.com/stackdriver/pricing#mgd-prometheus-pricing-summary

Support for ingestion via Google Cloud Managed Service for Prometheus should be implemented in metric starter
probbly similarly as it was done in OTEL: https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/googlemanagedprometheusexporter

Start by reading the metric starter's existing Stackdriver/Cloud Monitoring ingestion path, then compare it with the linked OpenTelemetry googlemanagedprometheusexporter. Done means the starter supports ingestion through Google Cloud Managed Service for Prometheus and has coverage for the new path.

Written by the indexing model from the issue text.