[Feature Request] Support Datadog Pod Autoscaler (DPA)

[eniko-dif]

Is your feature request related to a problem? Please describe.

We are shifting from Watermark Pod Autoscaler to DPA in Datadog and would like to use DPA in a WRT. Sadly, DPA does not follow HPA conventions, namely:

1. DPA uses targetRef instead of scaleTargetRef:
   spec: targetRef: apiVersion: apps/v1 kind: Deployment name: <your Deployment name>

2. DPA has a different (and complicated) metric setup that does not use matchLabels:
   `spec:
   targetRef:
   apiVersion: apps/v1
   kind: Deployment
   name: test
   ...
   objectives:

   • type: CustomQuery
     customQueryObjective:
     query:
     formulas:
     - "query1 / query2"
     queries:
     -
     name: query1
     dataSource: metrics
     metrics:
     query: "avg:system.cpu.user{*}"
     -
     name: query2
     dataSource: apm_metrics
     apm_metrics:
     stat: "latency_avg"
     service: "my-service"
     query_filter: "account:prod"`

If we want to use DPA with the Worker Controller, we'd need a way to template in the temporal_worker_deployment_name, the temporal_worker_build_id and the temporal_namespace for metric filters or queries.

Describe the solution you'd like

DPA can use resource-based targets which would not need the matchLabels setup, so as a first request, I'd like to ask for support for the targetRef field alongside the scaleTargetRef.

In the long term, it would be nice to be able to use queries with the additional fields templated in by the controller.