Prometheus should print appropriate error if `threshold` is not specifed

[zroubalik]

Report

threshold field is mandatory for Prometheus scaler, but we don't check that has been correctly specified.

Expected Behavior

Error message stating that threshold is not specified.

Actual Behavior

│ 1.64797501084928e+09    ERROR    controller.scaledobject    Reconciler error    {"reconciler group": "keda.sh", "reconciler kind": "ScaledObject", "name": "besu-non-val-node-scaledobject", "namespace" │
│ : "besu-non-val-node", "error": "HorizontalPodAutoscaler.autoscaling \"keda-hpa-besu-non-val-node-scaledobject\" is invalid: spec.metrics[0].external.target.averageValue: Invalid value: resource.Quant │
│ ity{i:resource.int64Amount{value:0, scale:0}, d:resource.infDecAmount{Dec:(*inf.Dec)(nil)}, s:\"0\", Format:\"DecimalSI\"}: must be positive"}

Steps to Reproduce the Problem

1. Deploy ScaledObject with Prometheus scaler that is missing threshold filed

Logs from KEDA operator

example

KEDA Version

2.6.1

Kubernetes Version

No response

Platform

No response

Scaler Details

Prometheus

Anything else?

No response