Allow ActiveSeriesRequest to return only native histograms and
include active bucket count in the label __nh_bucket_count__

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

Labels are already packed due to tag==stringlabels in TSDB so Add()
doesn't work on them. And after they are decoded for the response we'd
have to insert the bucket count label into the correctly sorted position
which would mean never being able to just send the encoded labels.
Instead of adding a new interface I'll add a backwards compatible array
of integers as a compromise.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>