Is your feature request related to a problem? Please describe.
It would be nice to see totals (or some other calculation) of all the series values within an x-value.

Describe the solution you'd like
Easiest way is to provide a way to pass a value / string to the tooltip header and it places it similar to the way the tooltip body handles the key/value pairs.

Additional context

Checklist

• this request is checked against already exist requests
• [ ] every related Kibana issue is listed under Kibana Cross Issues list
• [ ] kibana cross issue tag is associated to the issue if any kibana cross issue is present