DOCS-15137 update category information (#728)

Author: Dave

source/reference/command/serverStatus.txt

@@ -5097,6 +5097,11 @@ metrics
 
 .. serverstatus:: metrics.operatorCounters
 
+   A document that reports on the use of :ref:`aggregation pipeline
+   <aggregation-pipeline>` operators and expressions.
+
+.. serverstatus:: metrics.operatorCounters.expressions
+
    A document with a number that indicates how often
    :ref:`aggregation-expression-operators` ran.