In the metrics.h documentation the target to include a default
implementation of metrics was referring to the previous build system
(gyp). Now it is updated to refer to the current target.
BUG=None
NOTRY=True
TBR=henrika@webrtc.org
Review-Url: https://codereview.webrtc.org/2699093002
Cr-Commit-Position: refs/heads/master@{#16766}