Metrics API scaler doesn't parse Prometheus metric with multiple labels

[JorTurFer]

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Metrics API Scaler supports reading values in Prometheus format. The problem is that the Prometheus metric parse is just a strings comparison. This works for metrics without labels or just with a single label, but metrics with 2 or more labels are incorrectly parsed, having more fields than the expected amount of fields:

We should parse the Prometheus metrics and then compare the labels to avoid weird behaviors and the requirement of weird escape of the strings

Expected Behavior

Metrics API scaler supports any Prometheus metrics

Actual Behavior

It only supports metrics without labels or with a single label

Steps to Reproduce the Problem

This can be reproduced just updating the unit test TestGetValueFromResponse to include a Prometheus response with a metric that has at least 2 labels. e.g:

inputPrometheus := []byte(`# HELP backend_queue_size Total number of items
# TYPE backend_queue_size counter
backend_queue_size{queueName="zero"} 0
backend_queue_size{queueName="one"} 1
backend_queue_size{queueName="cache_enabled", instance="qa-khoffman-develop"} 3`)