Find a way for the `system-log` input to report the correct `input.type` when it delegates to other inputs

[belimawr]

The system-logs input is a "proxy input" that decides whether journald or log should be used as input for Filebeat's system module. At the moment of writing it is intended to be used only by the system integraiton.

One problem with the current implementation is that the input.type set in the event is system-logs instead of the actual name of the input running/collecting the data.

As a quick fix, we're overwriting input.type using ES ingest pipelines, however this is not ideal. This issue is about finding a way to get this fixed within Filebeat and have input.type correctly set.

• Relates: https://github.com/elastic/beats/issues/40526
• Relates: https://github.com/elastic/beats/pull/41246

[elasticmachine]

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

[pierrehilbert]

Following the revert here: https://github.com/elastic/beats/pull/41489 Should we keep this one open?

[belimawr]

We haven't deleted the input yet, we just reverted its usage, I'd keep it open for now, at least until we decide how we're handling journald in Filebeat's system module.