The custom values are the "le" bucket boundaries of native histograms
with custom buckets. They are never modified. It is ok to not copy them
when iterating a chunk, just reference them.
If we will ever have a function that modifies the custom values, like
'trim' for example. That function will have to make a copy on write.
Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>
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