Improve logs panel compatibility with configured log level field

The Grafana logs panel determines the log level from a level field, or falls back to looking for keywords in the message.
When using a different field name for the log level, all entries appear grey.

The Elasticsearch datasource works around this by renaming the logLevelField to level, which was removed here in f4a752e.
I understand why it was removed, it doesn't really seem like the best way of handling this. Especially when there could be another field named level that gets obscured because of this.

To make the logs panel work with the logLevelField option, would you consider making this behaviour opt-in?

Alternatively, it could probably also be done client-side. There should already be something handling this for the log volume panel in the explore tab, I'm not sure if the logs panel can be customized in the same way.

I'd be happy to create a PR for this, just let me know which option you'd prefer.

Start by tracing the Grafana logs panel handling for the configured logLevelField and review commit f4a752e, including the Elasticsearch datasource workaround. Compare it with the existing log volume panel customization mentioned in Explore. Done means entries use a non-level field for log severity without obscuring another level field, with the chosen behavior covered by appropriate tests.