Allow setting higher concurrency for loki sink

[jszwedko]

Requested by user: #6041 (comment)

@jszwedko As part of #5973, the request.concurrency configuration was disabled as a workaround to eliminate out-of-order issues. Although this eliminates out-of-order issues in Loki, it also reduces overall throughput. You made a review comment stating:

Instead of hard-coding to 1, maybe we could still let users set it if they really know what they are doing, but output a warning?

I would preference this to be a warning instead of a hard-coded value. Perhaps this should be included as part of the documentation rather than limiting the performance of vector?

In addition, there is progress in Loki to provide some leniency around out-of-order log submissions.

Is there a chance we could get request.concurrency re-enabled with a warning and/or documentation for users who may not understand the implications of increasing this value?

We should still do #6041 too, but this would be an easier fix to unblock users that are able to manage the higher concurrency.