Support grouping partial lines from Docker's journald log driver

[matthughes]

Describe the enhancement:

Docker's journald log driver (https://docs.docker.com/config/containers/logging/journald/) splits long log entries over multiple journald entries. If a message is being split, the driver at least helpfully annotates the journald entry with a CONTAINER_PARTIAL_MESSAGE:true flag.

This has been discussed here: https://github.com/mheese/journalbeat/issues/37 and a similar issue has been fixed in filebeats here: #6967.

It would be nice if journalbeats could look for this flag, and if present, group subsequent messages together into a single message prior to sending to the targeted output.

Describe a specific use case for the enhancement or feature:

Many folks use Docker and journald logging driver.

[botelastic]

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[botelastic]

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