metricName shouldn't be required and should be deprecated for Prometheus Scaler

[JorTurFer]

Proposal

Currently, Prometheus Scaler requires a metricName which is used only for KEDA internal things (it's not sent to the prom server).
This requirement isn't necessary since we introduced mechanisms to ensure the unique metrics names inside the ScaledObject and we should make it optional and also deprecate it in favor of self-generated internal name

https://github.com/kedacore/keda/blob/main/pkg/scalers/prometheus_scaler.go#L128-L132

[OsamaNabih]

Hey there,
I'm new here and think this might be a good issue to start with.
Could you elaborate more on the self-generation of the internal name? Is there code that does already, or will it be implemented in the PR? Does it follow a specific format?

Here's my progress so far, as noted by the comments at:
#4227
#4220
#3596 (comment)

--- a/pkg/scalers/prometheus_scaler.go
+++ b/pkg/scalers/prometheus_scaler.go
@@ -82,7 +82,7 @@ func NewPrometheusScaler(config *ScalerConfig) (Scaler, error) {  

        logger := InitializeLogger(config, "prometheus_scaler")

-       meta, err := parsePrometheusMetadata(config)
+       meta, err := parsePrometheusMetadata(config, logger)
        if err != nil {
                return nil, fmt.Errorf("error parsing prometheus metadata: %w", err)
        }
@@ -110,7 +110,7 @@ func NewPrometheusScaler(config *ScalerConfig) (Scaler, error) {
        }, nil
 }

-func parsePrometheusMetadata(config *ScalerConfig) (meta *prometheusMetadata, err error) {
+func parsePrometheusMetadata(config *ScalerConfig, logger logr.Logger) (meta *prometheusMetadata, err error) {
        meta = &prometheusMetadata{}

        if val, ok := config.TriggerMetadata[promServerAddress]; ok && val != "" {
@@ -148,6 +148,7 @@ func parsePrometheusMetadata(config *ScalerConfig) (meta *prometheusMetadata, er

        if val, ok := config.TriggerMetadata[promMetricName]; ok && val != "" {
                meta.metricName = val
+               logger.Info("WARNING: the field 'metricName' has been deprecated and will be removed in v2.12")
        }

        if val, ok := config.TriggerMetadata[promNamespace]; ok && val != "" {```

[JorTurFer]

Hi,
I'm not totally sure, but I think that there is another PR in progress that includes this
#4235

[OsamaNabih]

I see.

[zroubalik]

will be handled by #4220