Add sparse histograms to the metrics library and metrics client.

Samples to these histograms are any 32-bit int value.

BUG=chromium:222189
TEST=manual

Change-Id: Ic8d5773d05d717a275c4a4b5616e0e4c307337b8
Reviewed-on: https://gerrit.chromium.org/gerrit/45897


Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>