[Metrics UI] Graph per multiple fields in Metrics Explorer

[sorantis]

Metrics Explorer supports one group-by option, which limits the amount of information the user can browse at the same time. A workaround could be cumbersome:

1. Add a field to graph per.
2. Add a filter from one of the resulting charts.
3. Add a second field to graph per.

While this workaround can work for one instance, e.g. host, it won't help when this data needs to be compared across multiple instances/hosts.

Add the ability to graph per multiple fields to Metrics Explorer.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[sgrodzicki]

#66503