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

Instead of  checking non thread safe slice capacity

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

mdIdx was redundant when len(exemplars)>1, so got rid of it, rIdx
is enough.

Don't compare timestamp of incoming exemplar to timestamp of
minimal distance exemplar. Most of the time the incoming exemplar
will be newer. And if not, the previous code just replaced an
exemplar one index after the minimal distance exemplar. Which had
an index out of range bug, plus is essentially random.

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

# Conflicts:
#	prometheus/histogram.go

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