Fix Prometheus histograms when keylabels and values sit at the same path

[odacremolbap]

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]

Thank you for taking this @odacremolbap! please update the description to reflect the fix, also left a comment that should simplify the code

[exekias]

Well done on new tests and debugging this issue!

[ruflin]

Thanks for fixing this. The part I'm surprised is that this did not have an affect on any of the generated files from the new data generator. Would be great to also add an example with such a case to one of the metricsets.

[odacremolbap]

CoreDNS seems to be the first module affected by this.
Yes, it includes the case at generated golden files, not at data.json since it only selects one event and dismiss any of the others being generated. I was told that file was used for documentation purposes, so it makes sense to avoid many events from filling the docs.

[ruflin]

Only the data.json will only get one event, but the generated golden files for the tests should still contain all the data, see https://github.com/elastic/beats/tree/master/metricbeat/module/coredns/stats/_meta/testdata So if CoreDNS is affected in some examples, tests should fail.

[exekias]

afaik only the open PR is affected?