Is your feature request related to a problem? Please describe.
The current legend extra parameter is used to display the latest value on the data series, or the hovered data point value. We should make this configurable to show other values or statistics, like the total value, averages etc

Describe the solution you'd like
We should move the legend props to it's own <Legend> component and allow the user to customize in some way the possible extra parameter

Describe alternatives you've considered
n/a

Additional context
continuation of #246

Kibana Cross Issues

• [Lens] show max and min in charts' legend like Splunk does kibana#19485
• Metrics values in chart’s legend kibana#16096
• legend tooltip should display metrics kibana#10524
• Visualizations and dashboards need basic result count information kibana#6279

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