Docs for VSA cycle time and lead time

This adds a section to the Value Stream Analytics documentation to
explain how the lead time and cycle time metrics are calculated.

doc/user/analytics/value_stream_analytics.md

@@ -59,9 +59,20 @@ GitLab provides the ability to filter analytics based on a date range. To filter
 1. Optionally select a project.
 1. Select a date range using the available date pickers.
 
-## How the data is measured
+## How Time metrics are measured
 
-Value Stream Analytics records cycle time and data based on the project issues with the
+The "Time" metrics near the top of the page are measured as follows:
+
+- **Lead time**: median time from issue created to issue closed.
+- **Cycle time**: median time from first commit to issue closed.
+
+Note: A commit is associated with an issue by [crosslinking](../project/issues/crosslinking_issues.md) in the commit message or by manually linking the merge request containing the commit.
+
+![Value stream analytics time metrics](img/vsa_time_metrics_v13_0.png "Time metrics for value stream analytics")
+
+## How the stages are measured
+
+Value Stream Analytics records stage time and data based on the project issues with the
 exception of the staging and total stages, where only data deployed to
 production are measured.