[cc/metrics] Add a DIR_METADATA for cc/metrics.

The bugs for cc/metrics currently get assigned under the incorrect
component (Internals>Compositing). So add a DIR_METADATA for the
directory with the correct component (Internals>GPU>Metrics).

BUG=none

Change-Id: I63cd83b265938d21a5807bc1e7e0428e514b50c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499202
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821707}

cc/metrics/DIR_METADATA

+monorail {
+  component: "Internals>GPU>Metrics"
+}