[wip] JFR Event Streaming Support

[eiiches]

resolves #21

• [x] Support custom meters
• [x] Event sources
  • [ ] ~JMX notification~ I don't need this when I can just use JFR events. Maybe in another PR in future.
  • [x] JFR event streaming (>= Java 14)
  • [x] Subscribing to multiple event types at once in a single rule (Regex matching)
• [ ] Add examples
  • [ ] ZGC Allocation Stall metrics (JFR Event)
  • [ ] ZGC pause time metrics (JFR Event)
  • [ ] GC time histogram (~JMX Notification~ JFR Event)
  • [ ] Allocation Rate (JFR Event)
  • [ ] Reorganize commits
  • [x] Update README