Native histograms: optimize chunk iterator usage

[duricanikolic]

What this PR does

Prometheus introduced a possibility to optionally reuse the memory area of the given histogram or float histogram to reduce allocation (prometheus/prometheus#13340). We highlight the functions from the chunkenc.Iterator interface, that are of particular importance for the above mentioned change:

type Iterator interface {
	...
	// AtHistogram returns the current timestamp/value pair if the value is a
	// histogram with integer counts. Before the iterator has advanced, the behaviour
	// is unspecified.
	// The method accepts an optional Histogram object which will be
	// reused when not nil. Otherwise, a new Histogram object will be allocated.
	AtHistogram(*histogram.Histogram) (int64, *histogram.Histogram)
	// AtFloatHistogram returns the current timestamp/value pair if the
	// value is a histogram with floating-point counts. It also works if the
	// value is a histogram with integer counts, in which case a
	// FloatHistogram copy of the histogram is returned. Before the iterator
	// has advanced, the behaviour is unspecified.
	// The method accepts an optional FloatHistogram object which will be
	// reused when not nil. Otherwise, a new FloatHistogram object will be allocated.
	AtFloatHistogram(*histogram.FloatHistogram) (int64, *histogram.FloatHistogram)
}

Before #7219, which allowed Mimir to use Prometheus' optimizations mentioned above, Mimir's querier/iterator.chunkIterator used to implement Prometheus' chunkenc.Iterator, but the signatures of querier/iterator.chunkIterator's AtHistogram() and AtFloatHistogram() were not enriched with the optional *histogram.Histogram and *histogram.FloatHistogram arguments in #7219. This PR fixes that problem, so that querier/iterator.chunkIterator is an implementation of chunkenc.Iterator again.

Moreover, this PR ensures that Mimir's querier/iterator.chunkIterator and querier/iterator.chunkMergeIterator, both implementations of the chunkenc.Iterator interace, reuse histograms and float histograms memory, analogously to what is done in Prometheus.

Which issue(s) this PR fixes or relates to

Part of #7235

Checklist

• Tests updated.
• [na] Documentation added.
• [na] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [na] about-versioning.md updated with experimental features.

[krajorama]

LGTM