[receiver/mongodbatlas] Scraping logs fails for version 4.2 clusters

[BinaryFissionGames]

Describe the bug
MongoDB Atlas supports a range of cluster versions, with the lowest version being 4.2. This version does not share the same JSON log format as the other versions; It uses an unstructured log format (see: https://www.mongodb.com/docs/v4.2/reference/log-messages/). This causes scraping to fail, as the log message cannot be parsed.

Steps to reproduce
Attempt to scrape any 4.2 cluster.

What did you see instead?
This error and similar shows up:

invalid character 'T' looking for beginning of value

What version did you use?
v0.59.0

[BinaryFissionGames]

I'll be taking a look at supporting 4.2.

[github-actions]

Pinging code owners: @zenmoto. See Adding Labels via Comments if you do not have permissions to add labels yourself.