Updating system_wrappers/include/metrics.h docs
Updating system_wrappers/include/metrics.h docs

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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16766}

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