[metricbeat] support basepath in beat module

[stuartnelson3]

What does this PR do?

Support basepath in the beat module, as per the documentation

Why is it important?

This is expected behavior, and it's documented as being supported.

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.

How to test this PR locally

Related issues

• closes [metricbeat] basepath ignored/erased in beat module #28159

[mergify]

This pull request does not have a backport label. Could you fix it @stuartnelson3? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2021-09-28T15:33:39.144+0000

• Duration: 92 min 36 sec

• Commit: 8893ad7

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Test	Results
Failed	0
Passed	9145
Skipped	2413
Total	11558

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[simitt]

LGTM - but let's wait for someone from the agent team to review.

[sayden]

Hi 🙂

This is a screencapture from Wireshark of Beat module in the current state (master) without PR.

And this is my Beat config:

# Module: beat
# Docs: https://www.elastic.co/guide/en/beats/metricbeat/master/metricbeat-module-beat.html

- module: beat
  basepath: "/processes/metricbeat-default"
  metricsets:
    - stats
    - state
  period: 10s
  hosts: ["http://localhost:5066"]

Are we talking about the same thing? Because it seems to me that it's working as expected 🙂

[stuartnelson3]

That is what we're talking about, but it contradicts what I found when adding some logging (see #28159), and the tests I wrote confirm the original behavior strips the basepath, as well.

Check your wireshark capture further, I'm guessing there are also requests sent to localhost:5066 and localhost:5066/state.

Once I implemented my changes, it was successfully scraping the basepath and no longer 404'ing.

[sayden]

Jeeezzz I was getting crazy because I really couldn't see anything wrong on wireshark but I now have seen in the code a second call.

My test wasn't trying that call because I was already getting the 404 so the code-path was unreachable. Just because I was lazy to setup an nginx.

Good catch! Thank you for the explanation and the contribution. I leave you the "honor" of merging 😄