Use "personally identifiable information" instead of "PII"
on metrics.README.md.

Tbr: tandrii@chromium.org
Bug: None
Change-Id: I5294ab3ddfbc9e6705085a68751a477073cea052
Reviewed-on: https://chromium-review.googlesource.com/c/1337515Reviewed-by: Alan Bram <flyboy@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>