[8.x](backport #40558) [Journald] Restart journalctl if it exits unexpectedly

[mergify]

Proposed commit message

If journalctl exits unexpectedly the journald input will restart it and set the cursor to the last know position. Any error/non zero return code is logged at level error.

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

• Test if this PR fixes Filbeat stops shipping journald logs when encountered "failed to read message field: bad message" error #32782

How to test this PR locally

1. Build Filebeat from this PR and start it with the following configuration file:

filebeat.yml

filebeat.inputs:
  - type: journald
    id: my-unique-input-id
    enabled: true
    syslog_identifiers:
      - "potato"

output:
  console:
    pretty: true

2. Publish some events with the syslog identfier potato

echo "$(date '+%Y-%m-%d %T') - foo"| systemd-cat -t potato

3. Wait to see the output in the console
4. Kill journalctl

killall journalctl

5. Wait journalctl to restart, look for a log entry like this:

{
  "log.level": "info",
  "@timestamp": "2024-08-20T12:41:29.682-0400",
  "log.logger": "input.journald.reader.journalctl-runner",
  "log.origin": {
    "function": "github.com/elastic/beats/v7/filebeat/input/journald/pkg/journalctl.Factory",
    "file.name": "journalctl/journalctl.go",
    "file.line": 120
  },
  "message": "journalctl started with PID 3119456",
  "service.name": "filebeat",
  "id": "filestream-input-id",
  "input_source": "LOCAL_SYSTEM_JOURNAL",
  "path": "LOCAL_SYSTEM_JOURNAL",
  "ecs.version": "1.6.0"
}

6. Publish a few more messages and ensure they appear in the console

echo "$(date '+%Y-%m-%d %T') - foo"| systemd-cat -t potato

Related issues

• Closes [Journald] input crashes with "failed to read message field: cannot allocate memory" #39352

Tests

I run a test overnight where I used a mock to simulate constant failures and restarts of journalclt and monitored the host and Filebeat process to ensure there weren't any goroutine/resouce leaks. The screenshot below shows the CPU and memory usage from Filebeat as well as counters for the log entries stating journalctl crashed and was restarted.

## Use cases
## Screenshots
## Logs

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This is an automatic backport of pull request #40558 done by [Mergify](https://mergify.com).

[elasticmachine]

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