Introduce Google Cloud Platform (GCP) Stackdriver scaler

[tomkerkhove]

Since you can only run one custom metric adapter we should introduce a Google Cloud Platform (GCP) Stackdriver scaler which is compatible with Stackdriver Adapter:

• Scaler Source: Google Cloud Platform (GCP) Stackdriver
• How do you want to scale: See adapter, but might align over multiple iterations
• Authentication: See adapter, but might align over multiple iterations

[zroubalik]

Interesting idea! We should keep in mind that Stackdriver exposes both Custom Metrics and External Metrics. KEDA is using/exposing just External Metrics.

[frank-berlin]

Other way would to extent the keda keda-metrics-adapter in a way, that all requests, what he can't handle get transparent proxied to another metrics-adapter.
With this there would be a workaround to the limitation of k8s metrics api .
This also allow a smooth migration on existing cluster with hpa usage to use keda.

[tomkerkhove]

Let's do both!

Would you mind opening an issue for your idea @frank-berlin?

[frank-berlin]

done see #763

[yolossn]

@tomkerkhove I would like to work on this. Can you assign this to me?

[tomkerkhove]

Great to hear, thank you!