[TSVB] missing legend items for percentile in band mode

[markov00]

Kibana version:
master

Describe the bug:
When using percentile with band mode, there is no relative element displayed in the legend.

Steps to reproduce:

1. Create a percentile aggregation with mode band and a fill of x percent.
2. the legend doesn't show any item
3. using line mode the legend shows one element

Expected behavior:
If we have a band mode, I'm expecting to see 2 values on the legend: one for the higher percentile, and one for the lower percentile.

Screenshots (if relevant):

Errors in browser console (if relevant):
n/a

Any additional context:

@AlonaNadler maybe that was a previous decision to hide from the legend the last bucket for bands but I'm not aware of any issue on github related to that.

@elastic/kibana-tsvb-external I've open this bug to track that, but it will be solved through the elastic-chart replacement

[elasticmachine]

Pinging @elastic/kibana-app

[timroes]

Needs check if it's still valid after #58840 is fixed.