Add alexmt as owner of ad_tagging ct benchmark

Uncomments per-file owner now that alexmt@ is a committer

Change-Id: I501de30a9afde3c7a758c8957a6923dd3149829a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481703Reviewed-by: Ravi Mistry <rmistry@chromium.org>
Commit-Queue: Ravi Mistry <rmistry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818574}

tools/perf/contrib/cluster_telemetry/OWNERS

 rmistry@chromium.org
 
-# Uncomment when alexmt@ becomes a committer.
-# per-file ad_tagging_ct.py=alexmt@chromium.org
+per-file ad_tagging_ct.py=alexmt@chromium.org
 per-file ad_tagging_ct.py=johnidel@chromium.org
 per-file loading_ct.py=tdresser@chromium.org
 per-file loading_ct.py=maxlg@chromium.org