[Feature Request] Support Watermark Pod Autoscaler (WPA)

[eniko-dif]

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

The WPA scaler is similar to HPA in its spec, however there is a small field name difference for matchLabels:

HPA: spec.metrics[*].external.metric.selector.matchLabels
WPA: spec.metrics.external.metricSelector.matchLabels

This is blocking Datadog from using WPA with the WorkerResourceTemplate to scale workers.

Describe the solution you'd like

Support the WPA path to populate the matchLabels with the injected values in WRTs.
(Or optionally, a more general solution would be to support matchLabel detection for anything below spec.metrics.)