Fix for actor count metrics

[marcinbudny]

Description

• Fix for Metrics bug: Virtual Actor counter #1523
• prevent exception thrown in the Stopping handler from sending Restarting message to the actor before it's stopped

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)