Format tooltips more intuitively in split series scenarios

[flash1293]

For visualizations with multiple split series the tooltip isn't formatted in a straight forward way.

• There should be a separate row for each split (so bytes descending: 110 and machine.ram descending 6,442,...) or the label for the split series row should contain the name of all split series (e.g. bytes descending and machine.ram descending: 110 - 6,442,...)
• The rows should be ordered by the nesting indicated by the buckets in the left side panel. Metric should probably go last
• The label for the y axis should probably contain the used metric, not the series name ( Count of documents instead of 110 - 6,442,...)

[elasticmachine]

Pinging @elastic/kibana-app

[pinxtor]

Any updates on when this fix will be released?