metrics: tell what file we can't open on error

Use PDLOG and also log the filename.

BUG=chromium:467586
TEST=rebuild image and checked the log

Change-Id: I72f9838477adad71e79cc25f1f3a40bb9781b848
Reviewed-on: https://chromium-review.googlesource.com/260261


Tested-by: Stefan Sauer <ensonic@google.com>
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Stefan Sauer <ensonic@google.com>