Cherry-pick #11759 to 7.0: Fix Prometheus histograms when keylabels and values sit at the same path

[odacremolbap]

Cherry-pick of PR #11759 to 7.0 branch. Original message:

When a Prometheus histogram is configured to set KeyLabel at the same path at the event where the values will be written, labels will be smashed by values.

Those scenario patterns are:

metric_name{label1="value1",label2="value2"} metric_value 

with mappings defined as

&MetricsMapping{
	Metrics: map[string]MetricMap{
		"metric_name": Metric("metrics.name"),
	},
				Labels: map[string]LabelMap{
					"rank":  KeyLabel("metric.name.label1"),
					"alive": KeyLabel("metric.name.label2"),
				},

which will lead to labels not included for histograms

[exekias]

I think this is not affecting any module in 7.0 (I may be wrong)
do we really need to backport the change?

[odacremolbap]

I backported by inertia.
You are right. Closing.