investigate issues with ActorGroupMetrics (Scala 3)

[pjfanning]

ActorGroupMetricsSpec and ActorSystemMetricsSpec have some checks disabled when Scala 3 is used - long been the case

Also since Pekko 1.1, some ActorGroupMetricsSpec checks have had to be disabled for Scala 2.13. This is probably due to the use of Scala 2 inlining that is used in Pekko 1.1.

[pjfanning]

The Scala 2.13 issues should be sorted when Pekko 1.1.2 is released. apache/pekko#1485