Init histogram counters with 0

[mapno]

What this PR does:

Pushes a 0 to classic histogram's counter when the series is new to allow Prometheus to start from a non-null value.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[javiermolinar]

Question, maybe it makes sense to create a new ìnit` method in the interface:

type metric interface {
	name() string
        initMetrics(appender storage.Appender,...)
	collectMetrics(appender storage.Appender, timeMs int64, externalLabels map[string]string) (activeSeries int, err error)
	removeStaleSeries(staleTimeMs int64)
}

since I see we do the same logic in the counter

[mapno]

This logic is actually of the series, not the metric. It needs passing a couple of values, the function would actually encapsulate very little. I don't think it's worth the hassle.

func (co *counterSeries) init(appender storage.Appender, lbls labels.Labels, ts int64) {
	_, _ = appender.Append(0, lbls, ts, 0)
	co.registerSeenSeries()
}