Add Configurable metric_name_prefix

R=sergeyberezin@chromium.org
BUG=624470

Review-Url: https://codereview.chromium.org/2119933003
Cr-Mirrored-From: https://chromium.googlesource.com/infra/infra
Cr-Mirrored-Commit: db2a299c44a6a970cfb31c45669b1764eadee73d
4 files changed