Add comment to metrics.h
Add comment to metrics.h

BUG=None
NOTRY=True
TBR=rkaplow@chromium.org,asapersson@webrtc.org

Review-Url: https://codereview.webrtc.org/2557693002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15439}

ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc