Support filters to discard/modify events

- [ ] It should be possible to register filters via
- [ ] Public API
- [ ] Configuration for the case when developer prefers not to change applications source code
- [ ] Filter is called
- [ ] Right after an event is created
- [ ] Right after an event is ended (but before it's frozen and about to be added to the send-to-APM-server queue)
- [ ] Filter should be able
- [ ] To modify the event
- [ ] To discard the event
- [ ] Filter should be applied to all events - both the ones generated by automatic and manual instrumentation

[For example .NET Agent's filter's related API](https://www.elastic.co/guide/en/apm/agent/dotnet/current/public-api.html#filter-api)