doc: add basic documentation for CMAP event monitoring

Author: mbroadst

docs/reference/content/reference/management/cmap-monitoring.md

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+date = "2020-01-14T09:03:26-04:00"
+title = "Connection Pool Monitoring"
+[menu.main]
+  parent = "Management"
+  identifier = "CMAP"
+  weight = 100
+  pre = "<i class='fa'></i>"
++++
+
+# Connection Pool Monitoring
+
+The Node.js driver `3.5.0` or higher features Connection Pool Monitoring events, allowing an application or
+tool to monitor the internal workings of the driver's connection pool.
+
+**NOTE:** Connection pool monitoring is only available when the "Unified Topology" is enabled
+
+## Overview of CMAP events
+
+| Event | Description |
+| :----------| :------------- |
+| connectionPoolCreated | Emitted when a connection pool is created |
+| connectionPoolClosed | Emitted when a connection pool is closed, prior to server instance destruction |
+| connectionCreated | Emitted when a connection is created, but not necessarily when it is used for an operation |
+| connectionReady  | Emitted after a connection has successfully completed a handshake, and is ready to be used for operations|
+| connectionClosed | Emitted when a connection is closed |
+| connectionCheckOutStarted | Emitted when an operation attempts to acquire a connection for execution |
+| connectionCheckOutFailed | Emitted when an operation fails to acquire a connection for execution |
+| connectionCheckedOut | Emitted when an operation successfully acquires a connection for execution |
+| connectionCheckedIn | Emitted when a connection is returned to the pool after operation execution |
+| connectionPoolCleared | Emitted when the connection pool's generation count is increased |
+
+## Simple Code Example
+
+The following example demonstrates connecting to a replica set and printing out all CMAP related events:
+
+```js
+const MongoClient = require('mongodb').MongoClient;
+const url = 'mongodb://localhost:31000,localhost:31001/?replicaSet=rs';
+const client = new MongoClient(url);
+
+client.on('connectionPoolCreated', event => console.dir(event));
+client.on('connectionPoolClosed', event => console.dir(event));
+client.on('connectionCreated', event => console.dir(event));
+client.on('connectionReady', event => console.dir(event));
+client.on('connectionClosed', event => console.dir(event));
+client.on('connectionCheckOutStarted', event => console.dir(event));
+client.on('connectionCheckOutFailed', event => console.dir(event));
+client.on('connectionCheckedOut', event => console.dir(event));
+client.on('connectionCheckedIn', event => console.dir(event));
+client.on('connectionPoolCleared', event => console.dir(event));
+
+client.connect((err, client) => {
+  if (err) throw err;
+});
+```
+
+## Example Events
+
+### connectionPoolCreated
+```js
+ConnectionPoolCreatedEvent {
+  time: 2020-01-14T13:46:15.536Z,
+  address: 'localhost:31003',
+  options: { ... }
+}
+```
+
+### connectionPoolClosed
+```js
+ConnectionPoolClosedEvent {
+  time: 2020-01-14T13:54:53.570Z,
+  address: '127.0.0.1:34849'
+}
+```
+
+### connectionCreated
+```js
+ConnectionCreatedEvent {
+  time: 2020-01-14T13:54:53.579Z,
+  address: '127.0.0.1:34849',
+  connectionId: 1
+}
+```
+
+### connectionReady
+```js
+ConnectionReadyEvent {
+  time: 2020-01-14T13:54:53.579Z,
+  address: '127.0.0.1:34849',
+  connectionId: 1
+}
+```
+
+### connectionClosed
+```js
+ConnectionClosedEvent {
+  time: 2020-01-14T13:54:53.564Z,
+  address: '127.0.0.1:34849',
+  connectionId: 2,
+  reason: ...
+}
+```
+
+### connectionCheckOutStarted
+```js
+ConnectionCheckOutStartedEvent {
+  time: 2020-01-14T13:49:59.271Z,
+  address: 'localhost:31000'
+}
+```
+
+### connectionCheckOutFailed
+```js
+ConnectionCheckOutFailedEvent {
+  time: 2020-01-14T13:49:59.271Z,
+  address: 'localhost:31000'
+  reason: ...
+}
+```
+
+### connectionCheckedOut
+```js
+ConnectionCheckedOutEvent {
+  time: 2020-01-14T13:48:42.541Z,
+  address: 'localhost:31000',
+  connectionId: 1
+}
+```
+
+### connectionCheckedIn
+```js
+ConnectionCheckedInEvent {
+  time: 2020-01-14T13:48:42.543Z,
+  address: 'localhost:31000',
+  connectionId: 1
+}
+```
+
+### connectionPoolCleared
+```js
+ConnectionPoolClearedEvent {
+  time: 2020-01-14T13:58:11.437Z,
+  address: '127.0.0.1:45005'
+}
+```