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 <ekempin@google.com>
(cherry picked from commit f68598abd7fbc10dea5a86321bcd61682ae10cbe)

Documentation/metrics.txt

diff --git a/Documentation/metrics.txt b/Documentation/metrics.txt
index b6d6aad..1270971 100644
--- a/Documentation/metrics.txt
+++ b/Documentation/metrics.txt
@@ -86,6 +86,13 @@
 * `notedb/auto_rebuild_failure_count`: NoteDb auto-rebuilding attempts that
 failed by table.
 
+=== Reviewer Suggestion
+
+* `reviewer_suggestion/query_accounts`: Latency for querying accounts for
+reviewer suggestion.
+* `reviewer_suggestion/query_groups`: Latency for querying groups for reviewer
+suggestion.
+
 === Replication Plugin
 
 * `plugins/replication/replication_latency`: Time spent pushing to remote