Support filtering in Logs Volume panel for multiple log volumes

[harisrozajac]

We want to support filtering for multiple log volumes. This seems like a rare case because we haven't received any complaints, but it would make sense to implement it so filtering is enabled in Mixed datasource mode where logs come from two or more datasources.

The tasks are:

• Support filtering for multiple log volumes
• Make sure content outline works in this case

A place to start looking into this:

grafana/public/app/features/explore/Logs/LogsVolumePanelList.tsx

Lines 133 to 134 in e711d10

	// TODO: Support filtering level from multiple log levels
	onHiddenSeriesChanged={numberOfLogVolumes > 1 ? () => {} : onHiddenSeriesChanged}

)