Log error when statsd server fails to start and fix airflow docs

[belimawr]

Proposed commit message

This commit adds a log error when the statsd server fails to start, it
also improves the Airflow module documentation to make extra clear
that the module will start a statsd server.

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

How to test this PR locally

1. Create the following metricbeat.yml:

   metricbeat.modules:
   - module: airflow
     host: "1.2.3.4" # Any IP that does not belong to the host running Metricbeat
     port: "9102"
     ttl: "10s"
     period: 5s
     metricsets: [ 'statsd' ]
   
   output.console:
     enabled: true

2. Start Metricbeat
3. After a few seconds you should see a log error like this:

   {"log.level":"error","@timestamp":"2023-08-31T17:09:34.565+0200","log.logger":"airflow.statsd","log.origin":{"file.name":"server/server.go","file.line":201},"message":"could not start statsd server for module: airflow","service.name":"metricbeat","error":{"message":"failed to start UDP server: listen udp 1.2.3.4:9102: bind: cannot assign requested address"},"ecs.version":"1.6.0"}

## Related issues
## Use cases
## Screenshots
## Logs

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @belimawr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2023-09-12T06:31:58.485+0000

• Duration: 72 min 38 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4665
Skipped	1014
Total	5679

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[shmsr]

Looks good to me but I could see more adjustments that could be made to Airflow docs. Sample patch:

 This module collects metrics from
-https://airflow.apache.org/docs/apache-airflow/stable/logging-monitoring/metrics.html[Airflow metrics]. It runs a
-statsd server where airflow will send metrics to. The default metricset is `statsd`.
+https://airflow.apache.org/docs/apache-airflow/stable/logging-monitoring/metrics.html[Airflow metrics]. It runs a statsd server where airflow will send metrics to. The default metricset is `statsd`.
 
 [float]
 === Compatibility
 
-The Airflow module is tested with Airflow 2.1.0. It should work with version
-2.0.0 and later.
+The Airflow module is tested with Airflow 2.1.0. It should work with version 2.0.0 and later.
 
 [float]
 === Usage
-The Airflow module requires <<metricbeat-module-statsd,Statsd>> to receive Statsd metrics. Refer to the link for instructions about how to use Statsd.
+The Airflow module requires <<metricbeat-module-statsd,Statsd>> to receive statsd metrics. Refer to the link for instructions about how to use statsd.
 
-Add the following lines to your Airflow configuration file e.g. `airflow.cfg` ensuring `statsd_prefix` is left empty and replace `%METRICBEAT_HOST%` with the address metricbeat is running:
+Add the following lines to your Airflow configuration file e.g. `airflow.cfg` ensuring `statsd_prefix` is left empty and replace `%METRICBEAT_HOST%` with the address where metricbeat is running:

[shmsr]

Left one comment. Also, CI is failing. Could you please also run make update?

[belimawr]

@shmsr I added all your suggestions on e8dc1d6

[shmsr]

nitpick: rename the method receiver from b to m: s/b/m

[shmsr]

not important though

[belimawr]

That's a bigger change on the this file, I rather make it into a separated PR.

[belimawr]

The followup PR: #36591

I think when I first looked I was thinking about the opposite change 🤦‍♂️. It's a super small PR :D