metrics: fix lint warnings

This CL solve the cros lint warnings.
This is mostly due to:
* header guards
* include path
* few google style guide warnings

BUG=chromium:389229
TEST=FEATURES=test emerge-amd64-generic metrics

Change-Id: Ibbfcd2c88926bcc0c1ce9275b4ad0fb0748cd4de
Reviewed-on: https://chromium-review.googlesource.com/207248


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>