Customize display of a log line in the explore view

[fmassot]

Currently, it's possible to define a field to display a log line in the explore view.

It's not very flexible, and it would be nice to be able to customize it a bit more. We need to define some spec (and probably look at the last evolutions in Grafana explore view)

[fmassot]

We could let the user define a list of fields separated by commas in the "Message field name" parameter.
We could also let the user override this at query time in the explore view in the logs options.

what do you think @idrissneumann ?