Dashboard - Fix payload estimation in /add_histograms.
Dashboard - Fix payload estimation in /add_histograms.

Wasn't recording the first element properly.

TBR=eakuefner@chromium.org
Bug: catapult:#4118
Change-Id: Ief125bd29fafe291f0e762e26d060cd10dc48bbe
Reviewed-on: https://chromium-review.googlesource.com/841163
Reviewed-by: Simon Hatch <490a224d89645d3fea983d7189ce51bde0a00c1d@chromium.org>
Commit-Queue: Simon Hatch <490a224d89645d3fea983d7189ce51bde0a00c1d@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult