DOCS-14715 match expressions in serverStatus

Author: mungitoperrito

source/reference/command/serverStatus.txt

@@ -4905,7 +4905,10 @@ metrics
       },
       "operatorCounters" : {
             "expressions" : {
-               "<command>" : Long("0")
+               "<command>" : Long(<num>)
+            },
+            "match" : {
+               "<command>" : Long(<num>)
             }
       },
       "queryExecutor": {
@@ -5031,6 +5034,37 @@ metrics
 
    .. versionadded:: 5.0
 
+.. serverstatus:: metrics.operatorCounters.match
+
+   A document with a number that indicates how often
+   :ref:`match expressions <query-projection-operators-top>` ran. For
+   some operators, the number reported is a multiple of the times the
+   operator actually ran.
+
+   :ref:`Match expression operators <query-projection-operators-top>`
+   also increment as part of an aggregation pipeline :pipeline:`$match`
+   stage. If the ``$match`` stage uses the :query:`$expr` operator, the
+   counter for ``$expr`` increments, but the component counters do not
+   increment. 
+
+   Consider the following query: 
+
+   .. code-block:: javascript
+      :copyable: false
+
+      db.matchCount.aggregate(
+         [
+            { $match:
+               {  $expr: { $gt: [ "$_id", 0 ] } }
+            }
+         ]
+      )
+
+   The counter for ``$expr`` increments when the query runs. The
+   counter for ``$gt`` does not. 
+
+   .. versionadded:: 5.1
+
 .. serverstatus:: metrics.commands
 
    A document that reports on the use of database commands. The fields

source/release-notes/5.1.txt

@@ -87,9 +87,14 @@ General Improvements
 :dbcommand:`serverStatus` includes the following new fields in its
 output:
 
+Aggregation Metrics
+  - :serverstatus:`metrics.operatorCounters.match` A document which
+    indicates how often
+    :ref:`match expressions <query-projection-operators-top>` ran.
+
 Resharding Statistics
   - :serverstatus:`shardingStatistics.resharding.lastOpEndingChunkImbalance`
-  
+
 
 New Slot-Based Query Execution Engine
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -163,3 +168,4 @@ of the related projects.
    :hidden:
 
    /release-notes/5.1-compatibility
+