metrics: Spell out PII.

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/1337515
Reviewed-by: Alan Bram <flyboy@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

metrics.README.md

@@ -55,7 +55,8 @@ The metrics we're collecting are:
   - What host are we making the request to?
     Only collected for well known hosts like chromium-review.googlesource.com.
   - What path did we access on the server?
-    We map the path to an enum to make sure we're not collecting PII.
+    We map the path to an enum to make sure we're not collecting personally
+    identifiable information.
     i.e. we report 'changes/' instead of 'changes/12345'
   - What arguments were used on the request?
     We collect only known argument names, but not their values.