[FedCM] Move FedCmMetrics to content::webid and remove FedCm prefix

This CL renames the FedCmMetrics class to Metrics and removes the "FedCm"
prefix from the names of various enums inside the
content/browser/webid/metrics.h file.

This is done to improve clarity and reduce redundancy in the code.

Bug: 438743910
Change-Id: Ibc0753a8c34c69c5ea9b76ecfcf145d859544055
Bypass-Check-License: Renaming files, year should be kept intact
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6852559
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Sam Goto <goto@chromium.org>
Auto-Submit: Sam Goto <goto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503395}