[Metrics charts] UI changes for Metrics charts

[florent-leborgne]

Version: 9.2
PR: elastic/kibana#229815

Most likely place to update: https://www.elastic.co/docs/explore-analyze/visualize/lens#metric-trends

[alaudazzi]

CC @mariairiartef

[alaudazzi]

@mariairiartef as discussed during our meeting today, this is the doc page that needs to be updated https://www.elastic.co/docs/explore-analyze/visualize/lens#configure-the-visualization-components.
We also think that Docs #3086 related to elastic/kibana#233992 could be redundant. Please confirm.