[receiver/mongodbatlas] Raw log message is not the actual raw log message for JSON logs

[BinaryFissionGames]

What happened?

## Description
When receiving JSON logs, the mongodbatlas receiver decodes them, and throws away the original lines. It then "reconstructs" the original line by re-marshaling. This is inaccurate, and makes it more difficult to see if there was something missed in the model when it was marshalled (see #14177). 

The original log line is also present in two locations, as the body, and as an attribute (`raw_log`); It only really needs to be present in one place (the body).

## Steps to Reproduce
Scrape any 4.4+ mongodb cluster

## Expected Result
The original log line is present

## Actual Result
The original log line is not present, and any unknown fields are stripped.

Collector version

v0.60.0

Environment information

No response

OpenTelemetry Collector configuration

No response

Log output

No response

Additional context

No response

[github-actions]

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