metrics: fix remaining linter issues

Fixed various issues reported by cpplint.py when run on
src/platform2/metrics.

BUG=None
TEST=cpplint.py `find metrics/* | grep "\.cc\|\.h"`
     FEATURES=test emerge-link metrics

Change-Id: I589be238c97c38f985f24ffe26d2cdf259c96760
Reviewed-on: https://chromium-review.googlesource.com/210505


Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>