[Ingest Management] Report agent status during checkin

[michalpristas]

What does this PR do?

This PR adds a basic status reporter and controller structures used to compute overall agent health.
This computed status is then used during checkin and reported to fleet.

With each new configuration status is reset to healthy to avoid reporting failures from unrelated configurations.

Discussed with @nchaulet and for now it's ok not to reset long polling request in order to shorten delay in between status changes (some backpressure or rate limiter protection would need to be applied on either of the sides to prevent harms from frequent changes)

Why is it important?

related elastic/elastic-agent#120
related elastic/kibana#71009

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.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[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

• Build Cause: Pull request elastic/beats#23058 updated

• Start Time: 2020-12-15T08:01:20.986+0000

• Duration: 98 min 18 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17499
Skipped	1379
Total	18878

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	17499
Skipped	1379
Total	18878

[EricDavisX]

/package

[blakerouse]

Looks good to me.

I think more work will need to be done to link it to each input for the configuration. At the moment this works for 7.11 being that we do not have per-input status reporting.

[blakerouse]

Would be easier to do these in a defered.

defer func() {
    if err != nil {
        o.statusReporter.Update(status.Failed)
        return err
    }
}()

[michalpristas]

yes indeed