Fix the metric fields in mastermon FilePoller.
Fix the metric fields in mastermon FilePoller.

R=pgervais@chromium.org
BUG=524111

Review URL: https://codereview.chromium.org/1324433002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra