Update Device Defender demo documentation (#1574)

Author: aggarw13

docs/doxygen/demos/defender_demo.dox

@@ -12,14 +12,20 @@ The demo reports the following metrics to the AWS IoT Device Defender Service:
 - List of open TCP ports
 - List of open UDP ports
 - List of established connections
+- System CPU usage statistics (as a custom metric of number-list type)
+- System memory statistics (as a custom metric of string-list type)
 
-The demo gets the above metrics by reading `/proc/net/dev`, `/proc/net/tcp` and,
-`/proc/net/udp`. It then constructs a JSON report in the format as expected by
+The demo gets the above metrics by reading `/proc/net/dev`, `/proc/net/tcp`,
+`/proc/net/udp`, `/proc/uptime`, and `/proc/meminfo`. It then constructs a JSON report in the format as expected by
 the AWS IoT Device Defender service. The constructed JSON report is then
 published on the device defender MQTT topic reserved for publishing JSON
 reports. The demo then waits for a report accepted response, and ensures that
 the response contains the same report Id as was sent in the request.
 
+For the AWS IoT Device Defender service to make use of the custom metrics reported by the demo, you need to
+define the custom metrics in your account. You can use either the AWS console or CLI to create
+custom metrics in your account. For more information, please refer to [Custom Metrics AWS documentation](https://docs.aws.amazon.com/iot/latest/developerguide/dd-detect-custom-metrics.html).
+
 <div class="caption" style="text-align:center">
 AWS IoT Device Defender Demo Workflow
 </div>

docs/lexicon.txt

@@ -51,6 +51,7 @@ corehttp
 corejson
 coremqtt
 corepkcs
+cpu
 crontab
 crt
 csr
@@ -59,6 +60,7 @@ da
 datatypes
 daws
 dclient
+dd
 debian
 defgroup
 dev
@@ -112,6 +114,7 @@ lowercamelcase
 lowercamelcased
 mainpage
 matchtopic
+meminfo
 metadata
 misra
 mkdir
@@ -196,4 +199,4 @@ urls
 userguide
 wikipedia
 www
-xz
+xz