Extend Azure Monitor scaler to support custom metrics

[amirschw]

Add metricNamespace parameter to the Azure Monitor scaler to allow scaling based on custom metrics.

When querying Custom metrics in Azure Monitor, the metric namespace must be provided in the API request. Otherwise, only default metrics can be found. E.g. for Azure Key Vault, when trying to get a custom metric without specifying the namespace, we will get the following error from the metrics client:

keda_metrics_adapter/provider "msg"="error getting metric for scaler" "error"="error getting azure monitor metric QueueDepth: insights.MetricsClient#List: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="BadRequest" Message="Failed to find metric configuration for provider: Microsoft.KeyVault, resource Type: vaults, metric: QueueDepth, Valid metrics: ServiceApiHit,ServiceApiLatency,ServiceApiResult,SaturationShoebox,Availability"" "scaledObject.Name"="nginx-scaler" "scaledObject.Namespace"="keda" "scaler"={}

Documentation update PR: kedacore/keda-docs#468

Partly fixes #1489 (I don't think it covers the Application Insights metrics)

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• Tests have been added
• A PR is opened to update the documentation on (repo, PR) (if applicable)
• Changelog has been updated

Signed-off-by: amirschw 24677563+amirschw@users.noreply.github.com

[zroubalik]

LGTM