Expose filestream metrics together with log input

[belimawr]

What does this PR do?

The filestream input now also exposes the following metrics:

• filebeat.harvester.closed
• filebeat.harvester.open_file
• filebeat.harvester.started
• filebeat.harvester.running

Why is it important?

The filestream input did not expose those metrics, which is very confusing, especially for users migrating from the log input

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. Start Filebeat and ensure the HTTP endpoint is enabled, here is a config example:

filebeat.inputs:
  - type: filestream
    id: my-filestream-id
    enabled: true
    close.reader.after_interval: 30s
    paths:
      - /tmp/filestream*.log

output.console:
  codec.json:
    pretty: true

logging:
  to_stderr: true

http:
  enabled: true
  host: localhost
  port: 5066

2. Create files matching the glob /tmp/filestream*.log.
3. Check the metrics with curl -sL http://localhost:5066/stats | jq '.[].harvester | select(.!=null)'
4. Check the monitoring log metrics contains the correct values, look for entries like:

{"log.level":"info","@timestamp":"2023-06-20T18:36:14.044+0200","log.logger":"monitoring","log.origin":{"file.name":"log/log.go","file.line":187},"message":"Non-zero metrics in the last 30s","service.name":"filebeat","monitoring":{"metrics":{"beat":{"cgroup":{"cpu":{"id":"emacs.service"},"memory":{"id":"emacs.service","mem":{"usage":{"bytes":26226442240}}}},"cpu":{"system":{"ticks":20,"time":{"ms":20}},"total":{"ticks":30,"time":{"ms":30},"value":30},"user":{"ticks":10,"time":{"ms":10}}},"handles":{"limit":{"hard":524288,"soft":524288},"open":12},"info":{"ephemeral_id":"402cd7c0-145a-48fd-bda2-553f979af7a3","name":"filebeat","uptime":{"ms":30016},"version":"8.9.0"},"memstats":{"gc_next":11980576,"memory_alloc":7224144,"memory_sys":26326280,"memory_total":15859808,"rss":64663552},"runtime":{"goroutines":32}},"filebeat":{"events":{"active":0,"added":2,"done":2},"harvester":{"open_files":2,"running":2,"started":2}},"libbeat":{"config":{"module":{"running":0}},"output":{"events":{"acked":1,"active":0,"batches":1,"total":1},"type":"console","write":{"bytes":616}},"pipeline":{"clients":3,"events":{"active":0,"filtered":1,"published":1,"total":2},"queue":{"acked":1,"max_events":4096}}},"registrar":{"states":{"current":1,"update":1},"writes":{"success":1,"total":1}},"system":{"cpu":{"cores":16},"load":{"1":4.3,"15":3.85,"5":4.06,"norm":{"1":0.2688,"15":0.2406,"5":0.2537}}}},"ecs.version":"1.6.0"}}

Related issues

• Closes Expose file handles count for log and filestream inputs #33771

## Use cases
## Screenshots
## Logs

[mergify]

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If this is a bug or security fix, could you label this PR @belimawr? 🙏.
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[elasticmachine]

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• Start Time: 2023-06-21T14:14:01.148+0000

• Duration: 70 min 7 sec

Test stats 🧪

Test	Results
Failed	0
Passed	7987
Skipped	757
Total	8744

💚 Flaky test report

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[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[rdner]

@belimawr seems like some metric tests are failing now, can you have a look?

[michalpristas]

change looks ok,
tested also with closing files, works

[belimawr]

I removed the flaky test and will implement it on #35863.
I'm doing this to get it merged for the next release.

[pierrehilbert]

Was approved by @michalpristas before your remove the flaky test