Merge branch 'mw-add-mttm-to-mr-analytics-docs' into 'master'

Add MTTM to Merge request analytics docs

See merge request gitlab-org/gitlab!53956

doc/user/analytics/merge_request_analytics.md

@@ -39,6 +39,15 @@ Merge Request Analytics could be used when:
 
 The following visualizations and data are available, representing all merge requests that were merged in the given date range.
 
+### Mean time to merge
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/229389) in GitLab 13.9.
+
+The mean time to merge (MTTM) metric shows the average time between when a merge request is created,
+and when it is merged. To view how the MTTM changes over time, compare MTTM across different date ranges.
+
+![Mean time to merge](img/mr_mean_time_to_merge_metric_v13_9.png "Merge Request Analytics - MTTM metric showing the average time it takes from initiating a MR to being merged")
+
 ### Throughput chart
 
 The throughput chart shows the number of merge requests merged per month.