Fix update target in metricbeat magefile.go to include collectDocs target

[blakerouse]

What does this PR do?

Fixes the mage update target in metricbeat to also call mage collectDocs.

Why is it important?

So the docs/ are generate for metricbeat.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation~~
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Calling mage update updates the documentation.

How to test this PR locally

$ mage update

or

$ make update

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[kaiyan-sheng]

Thank you for fixing this!!

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request Fix update target in metricbeat magefile.go to include collectDocs target #18062 opened]

• Start Time: 2020-04-28T18:02:48.674+0000

• Duration: 65 min 38 sec (3877661)

• Commit: 7d753cb

Test stats 🧪

Test	Results
Failed	0
Passed	2634
Skipped	733
Total	3367

[dedemorton]

I think there are still problems (unless we removed support for make docs and no one told me). :-)

1 - make docs doesn't work.
2 - When I run make update, it doesn't update fields.asciidoc with changes made to the fields.yml.

Here's git status after running make update from beats/metricbeat. Notice that fields.yml changed, but the fields.asciidoc is not updated as it should be:

Rhodas-MBP:metricbeat⎇ metricbeat-collect-docs-on-update⇡$ git status
On branch metricbeat-collect-docs-on-update
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   module/aerospike/_meta/fields.yml
	modified:   module/aerospike/fields.go

[dedemorton]

I checked Filebeat and confirmed that make docs and make docs-preview do work for that beat, so it must be a metricbeat issue.

[dedemorton]

Seems like this commit broke the doc scripts for metricbeat: 8786d05#diff-34acb1c4bf062801dbb1a2e9554fcfaf

[blakerouse]

@dedemorton Nice catch, I have updated the magefile to do the correct thing for the asciidocs on fields. I also added the make doc and make doc-preview shims, so those will work now in the metricbeat directory.