Record spans dropped through event processors

### Description

As a follow up to https://github.com/getsentry/sentry-javascript/pull/13022, where we ensured to recored spans dropped in `beforeSendTranscation`, we should probably extend this to transactions dropped through event processors.

```[tasklist]
### Tasks
- [ ] Record if `event.spans.length` is lower after an event processor (indicating that it was dropped)
- [ ] Recording dropped spans if a whole transaction is dropped in an event processor
```