libbeat: log debug for proxy_url and fixed docs

[belimawr]

What does this PR do?

Fixes the documentation regarding Elasticsearch output proxy_url
configuration format.

Debug logs are added with the breakdown of proxy_url.

Why is it important?

Our documentation did not mach the way proxy_url is parsed for Elasticsearch output.

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

Add the proxy_url setting to any output that supports it, start a Beat with log debug, look for the log message breaking down the proxy URL that look like this:

{"log.level":"debug","@timestamp":"2022-04-04T18:05:42.644+0200","log.logger":"elasticsearch","log.origin":{"file.name":"elasticsearch/elasticsearch.go","file.line":68},"message":"breaking down proxy URL. Scheme: 'https', host[:port]: 'my-proxy.proxy:8888', path: '/proxy/path'","service.name":"filebeat","ecs.version":"1.6.0"}
{"log.level":"info","@timestamp":"2022-04-04T18:05:42.644+0200","log.logger":"elasticsearch","log.origin":{"file.name":"elasticsearch/elasticsearch.go","file.line":69},"message":"Using proxy URL: https://my-proxy.proxy:8888/proxy/path","service.name":"filebeat","ecs.version":"1.6.0"}

Configuration example:

output.elasticsearch:
  enabled: true
  hosts: ["localhost:9200"]
  proxy_url: "https://my-proxy.proxy:8888/proxy/path"

Related issues

• Closes proxy_url for Elasticsearch output (and possible others) is parsed wrong when scheme is not provided. #29249

## Use cases
## Screenshots
## Logs

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[elasticmachine]

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• Start Time: 2022-04-06T14:51:50.248+0000

• Duration: 52 min 15 sec

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Test	Results
Failed	0
Passed	16865
Skipped	1881
Total	18746

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[belimawr]

/test

[belimawr]

This is just a documentation and log debug PR, I won't fix the issues the linter is pointing out because they're were already there and I don't want to change the existing code at the moment.