Add description for pending tasks metric

Change-Id: Ie6b8b692c72dae09896263f54fc4232715154073

src/main/resources/Documentation/metrics.md

diff --git a/src/main/resources/Documentation/metrics.md b/src/main/resources/Documentation/metrics.md
index 55b50f7..ac1539c 100644
--- a/src/main/resources/Documentation/metrics.md
+++ b/src/main/resources/Documentation/metrics.md
@@ -34,6 +34,8 @@
 
 - `in_flight`: (gauge) number of tasks currently being executed.
 
+- `pending`: (gauge) number of tasks waiting to be executed.
+
 - `started`: (counter) number of tasks started.
 
 - `rescheduled`: (counter) number of tasks re-scheduled for execution.