metrics client: add basic sanity check for numeric arguments

"atoi" and "atod" don't do any conversions or error checks, they
just return 0 on error.  They should be illegal.

BUG=chromium:356932
TEST=checked various valid and invalid inputs

Change-Id: I53a92ccab5f44e18e8ac27154c2b2826d355a6c0
Reviewed-on: https://chromium-review.googlesource.com/191971


Reviewed-by: Darren Krahn <dkrahn@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>