[Docs] Update custom beat docs (elastic#22194)

* update custom beat docs

* add changelog

* update explanation

* Replace make with mage

CHANGELOG.next.asciidoc

@@ -188,6 +188,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - The `o365input` and `o365` module now recover from an authentication problem or other fatal errors, instead of terminating. {pull}21259[21258]
 - Orderly close processors when processing pipelines are not needed anymore to release their resources. {pull}16349[16349]
 - Fix memory leak and events duplication in docker autodiscover and add_docker_metadata. {pull}21851[21851]
+- Fixed documentation for commands in beats dev guide {pull}22194[22194]
 - Fix parsing of expired licences. {issue}21112[21112] {pull}22180[22180]
 
 *Auditbeat*

docs/devguide/create-metricset.asciidoc

@@ -32,20 +32,20 @@ The metricset that you created is already a functioning metricset and can be com
 +
 [source,bash]
 ----
-make collect
-make
+mage update
+mage build
 ----
 +
-The first command, `make collect`, updates all generated files with the most recent files, data, and meta information from the metricset. The second command,
-`make`, compiles your source code and provides you with a binary called metricbeat in the same folder. You can run the
+The first command, `mage update`, updates all generated files with the most recent files, data, and meta information from the metricset. The second command,
+`mage build`, compiles your source code and provides you with a binary called metricbeat in the same folder. You can run the
 binary in debug mode with the following command:
 +
 [source,bash]
 ----
 ./metricbeat -e -d "*"
 ----
 
-After running the make commands, you'll find the metricset, along with its generated files, under `module/{module}/{metricset}`. This directory
+After running the mage commands, you'll find the metricset, along with its generated files, under `module/{module}/{metricset}`. This directory
 contains the following files:
 
 * `\{metricset}.go`