Add Support for Stackdriver (CloudMonitoring) MetricTemplate with PromQL query

[russellrc-keebo]

Describe the feature

Google Cloud Monitoring supports PromQL besides MQL, by consuming the Prometheus HTTP API (https://cloud.google.com/stackdriver/docs/managed-prometheus/query#http-api-details)

What problem are you trying to solve?

• PromQL is a more widely adopted query language and writing queries in MQL is quite cumbersome.
• This would make it very easy to switch between Prometheus and Cloud Monitoring metric providers.
• For GCP users, this has the additional benefit to not have to expose a Prometheus FE service in order to make Flagger work.

Proposed solution

Extend the stackdriver metric provider implementation to use the prometheus api
https://cloud.google.com/monitoring/api/ref_v3/rest/v1/projects.location.prometheus.api.v1/query

What do you want to happen? Add any considered drawbacks.
N/A

Any alternatives you've considered?

• Either expose a Prometheus FE service (which consumes the metrics from Cloud Monitoring)
• Use MQL queries as per the Flagger documentation

Is there another way to solve this problem that isn't as good a solution?
N/A