Add structured logging to logs input

[urso]

• Enhancement

What does this PR do?

The logging of the logs input is updated to make use of structured
logging. Each logs input instance will create an unique ID, that will be
logged as "input_id".
The input and harvester will also add file state information like:
source, state_id, finished, os_id. The os_id holds the inode, and
state_id is the ID that is used internally by filebeat to track the
file.

Why is it important?

By filtering the logs for one or the other ID, one can track all
logs at a different level. Filtering by "state_id" allows us to monitor
all logs and state changes for a given file, even if the file is
renamed, closed and reopened.

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

Run filebeat with the logs input and debug logs enabled.

File state updates after a scan or harvester logs should log contextual information like the inode, state_id, harvester_id, input_id.

[elasticmachine]

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Build stats

• Build Cause: Pull request #25299 updated

• Start Time: 2021-04-27T20:36:45.338+0000

• Duration: 121 min 1 sec

• Commit: ba8fdde

Test stats 🧪

Test	Results
Failed	0
Passed	13707
Skipped	2285
Total	15992

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Test stats 🧪

Test	Results
Failed	0
Passed	13707
Skipped	2285
Total	15992

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[ph]

@urso is this something you want to add for 7.14?

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b fb-logs-log upstream/fb-logs-log
git merge upstream/master
git push upstream fb-logs-log

[urso]

is this something you want to add for 7.14?

The item is 'complete' with this PR, so yeah it is 7.14 :)
As this helps debugging potential issues (correlating logs), this change goes under the unbrella of supportability.