Mention chromium-metrics-reviews@google.com when recommending histograms.xml reviewers

This CL updates the comment in tools/metrics/histograms/OWNERS to
mention chromium-metrics-reviews@google.com when git cl upload suggests
a reviewer when touching histograms.xml.


Before CL:

Suggested OWNERS: (Use "git-cl owners" to interactively select owners.)
    owner@chromium.org is commented as:
      Use the following owners only if:
- You work in the same area as them.
- They are already a good reviewer for the non-metrics part of the CL.


After CL:

Suggested OWNERS: (Use "git-cl owners" to interactively select owners.)
    owner@chromium.org is commented as:
      Prefer chromium-metrics-reviews@google.com instead.
Use the following owners only if:
- You work in the same area as them.
- They are already a good reviewer for the non-metrics part of the CL.

Change-Id: I9162a55caac98f7481f8d2517acc509faee969b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418501
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809603}

tools/metrics/histograms/OWNERS

@@ -11,6 +11,7 @@ per-file histograms.xml=file://base/metrics/OWNERS
 # buckets.
 per-file enums.xml=*
 
+# Prefer chromium-metrics-reviews@google.com instead.
 # Use the following owners only if:
 #  - You work in the same area as them.
 #  - They are already a good reviewer for the non-metrics part of the CL.