Read journal entries from all boots

[belimawr]

Proposed commit message

Some versions of journalctl will only return messages from the current boot when --follow is passed, it will even ignore the cursor or date arguments.

This commit reads messages from all boots by first calling journalctl without the --follow flag, reading all entries and once it successfully exits, then we restart journalctl with the cursor and the --follow flag.

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.

## Disruptive User Impact

Author's Checklist

• Ensure TestInputParsers from filebeat/input/journald/input_parsers_test.go is not flaky
• Add tests to ensure messages from all boots are read
• Check whether the restart of journalclt can cause problems with multiline parser (see Filebeat's journald input might return an empty message that breaks multiline parser #41331)

How to test this PR locally

1. Run the tests

cd filebeat/input/journald
go test -run=TestInputCanReadAllBoots

2. Run Filebeat reading filebeat/input/journald/testdata/multiple-boots.journal

There must be 6 entries, you can see the plaintext entries by looking at filebeat/input/journald/testdata/multiple-boots.export or by running:

journalctl --file filebeat/input/journald/testdata/multiple-boots.export

3. Fully manual test

1. Run the journald input on a machine (or a journal file) that has got messages from more than one boot
2. Ensure all messages in the journal are correctly ingested.

Related issues

• Closes Journald input only ingests events from the current boot #41083

## Use cases
## Screenshots
## Logs

[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 41083-journald-input-all-boots upstream/41083-journald-input-all-boots
git merge upstream/main
git push upstream 41083-journald-input-all-boots

[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-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[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 41083-journald-input-all-boots upstream/41083-journald-input-all-boots
git merge upstream/main
git push upstream 41083-journald-input-all-boots

[elasticmachine]

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

[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 41083-journald-input-all-boots upstream/41083-journald-input-all-boots
git merge upstream/main
git push upstream 41083-journald-input-all-boots

[rdner]

A few linter issues that I think we should fix to have an easy win.