Display y0 and y1 values on legend for band charts

[markov00]

Describe the bug
The current implementation visualize only one last bucket value on the legend. We have to display either the y0 and the y1 on the legend

To Reproduce
Steps to reproduce the behavior:

1. Create an area band chart with one series
2. The legend will display only one value, not the y0 and y1

Expected behavior
The legend should display either y1 value (the default) and the y0 value if present and configured on the dataset.

Screenshots

Version (please complete the following information):

• OS: all
• Browser: all
• Elastic Charts: master/ 3.10.0

Additional context
n/a

Errors in browser console
n/a

Kibana Cross Issues

• [TSVB] missing legend items for percentile in band mode  kibana#34943
• [Logs UI] Change tool tip labels for expected boundaries kibana#45297

Checklist

• 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