Note: This is deliberately an incompatible change. Since we have never
used histograms in remote read/write yet, there is no point in keeping
compatibility. This _is_, however, compatible to the state in the main
branch.
This commit flattens the bucket message into top-level fields. This
has the disadvantage of now having two triples of fields prefixed with
`negative_...` or `positive_...`. However, with this structure, we
save one tag on the wire. And, perhaps more importantly, we mirror the
structure of the `histogram.Histogram` Go type.
This commit also adjusts `repeated` fields to use names in the plural
form, as it is also the case for the fields that already existed.
This also adds a doc comment to `HistogramProtoToHistogram` and
changes its return type to a pointer (which is more convenient and
probably more efficient).
Signed-off-by: beorn7 <beorn@grafana.com>