Add description for in_flight metric

Change-Id: I25d073753b7d1c30944982dcefa5d51593747754

src/main/resources/Documentation/metrics.md

diff --git a/src/main/resources/Documentation/metrics.md b/src/main/resources/Documentation/metrics.md
index 055b2bd..55b50f7 100644
--- a/src/main/resources/Documentation/metrics.md
+++ b/src/main/resources/Documentation/metrics.md
@@ -32,6 +32,8 @@
 - `scheduled`: (counter) number of tasks triggered and scheduled for
   execution.
 
+- `in_flight`: (gauge) number of tasks currently being executed.
+
 - `started`: (counter) number of tasks started.
 
 - `rescheduled`: (counter) number of tasks re-scheduled for execution.