DOCSP-38042: CMAP Tracking Example (#891)

(cherry picked from commit 1859354)

Author: mcmorisi

source/code-snippets/monitoring/cpm-status.js

@@ -0,0 +1,32 @@
+function connectionPoolStatus(client) {
+  let checkedOut = 0;
+
+  function onCheckout() {
+    checkedOut++;
+  }
+
+  function onCheckin() {
+    checkedOut--;
+  }
+
+  function onClose() {
+    client.removeListener('connectionCheckedOut', onCheckout);
+    client.removeListener('connectionCheckedIn', onCheckin);
+
+    checkedOut = NaN;
+  }
+
+  // Decreases count of connections checked out of the pool when connectionCheckedIn event is triggered
+  client.on('connectionCheckedIn', onCheckin);
+
+  // Increases count of connections checked out of the pool when connectionCheckedOut event is triggered
+  client.on('connectionCheckedOut', onCheckout);
+
+  // Cleans up event listeners when client is closed
+  client.on('close', onClose);
+
+  return {
+    count: () => checkedOut,
+    cleanUp: onClose
+  };
+}

source/fundamentals/monitoring/connection-monitoring.txt

@@ -27,10 +27,10 @@ network handshakes your application needs to perform and can help your
 application run faster.
 
 The following sections demonstrate how to record connection pool events in your
-application or want to explore the information provided in these events.
+application and explore the information provided in these events.
 
-Event Subscription Example
---------------------------
+Event Subscription Examples
+---------------------------
 
 You can access one or more connection pool events using the driver by
 subscribing to them in your application. The following example demonstrates
@@ -40,6 +40,13 @@ pool monitoring events created by the MongoDB deployment:
 .. literalinclude:: /code-snippets/monitoring/cpm-subscribe.js
    :language: javascript
 
+Connection pool monitoring events can aid you in debugging and understanding
+the behavior of your application's connection pool. The following example uses connection
+pool monitoring events to return a count of checked-out connections in the pool:
+
+.. literalinclude:: /code-snippets/monitoring/cpm-status.js
+   :language: javascript
+
 Event Descriptions
 ------------------