feat: add metricscardinality to heartbeat

[alexcastilio]

Description

Add metricscardinality to heartbeat. This will give visibility on the number of time series being exposed by retina.

Related Issue

#1040

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Screenshots (if applicable) or Testing Completed

Metrics exported with heartbeat:

Additional Notes

Metrics of types histogram and summary expose multiple time series during a scrape. Code is counting according to number of time series exposed at /metrics endpoint.

Ref:
https://prometheus.io/docs/concepts/metric_types/#histogram
https://prometheus.io/docs/concepts/metric_types/#summary

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[matmerr]

I think a simpler solution may just be to enumerate over the metrics and take the length of the metrics slice

https://gist.github.com/matmerr/f2e31d9be0b5af0eec750fa944edc617#file-counter_test-go-L58

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// count all metrics and combination of labels currently exposed on
// prometheus endpoint
metrics, err := registry.Gather()
if err != nil {
	fmt.Println("Error gathering metrics: ", err)
}

// count all metrics exposed for each metric
var metriccounter int
for _, metric := range metrics {
	metriccounter += len(metric.Metric)
}

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where the metrics endpoint looks like this:

and in stdout we see the same, total metrics is 4