Give the mastermon 'up' metric a 'buildbot/' prefix to match the other metrics exported here.
Give the mastermon 'up' metric a 'buildbot/' prefix to match the other metrics exported here.

BUG=490572

Review URL: https://codereview.chromium.org/1172593002

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