Document metrics for reviewer suggestion
Document metrics for reviewer suggestion

These were added by change I53c1e0.

In this cherry-pick the documentation for the "load_accounts" and
"recommend_accounts" metrics is omitted because those were not
backported to stable-2.13.

Change-Id: Ia080f04bf214c6f8d996bf8bed706db690bf4140
Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
(cherry picked from commit f68598abd7fbc10dea5a86321bcd61682ae10cbe)

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