Proposal

Depends on #13487

In this feature we take advantage of the fact that custom bucket definition is never written and when loading vector selectors, the chunk iterator is reused. Thus the CustomValues []float64 field in histogram.Histogram, histogram.FloatHistogram, writev2.Histogram can be interned (passed by slice reference) and not copied all the time.

• Make sure there's a benchmark test for vector selector over custom histograms.
• The histogram and float histogram chunk iterator implementation should intern the custom buckets, that is set in the returned histogram as a slice reference.
• Implement that the chunk iterator reuses the previous value of customBounds when the iterator is reused.
• Verify with the benchmark / other means that customBounds is interned correctly.
• Investigate accepting samples via RW2.0 vs interning. Is there any opportunity to optimize?