 Report dropped spans in client reports

### Description

Meta issue: https://github.com/getsentry/projects/issues/70

```[tasklist]
### Blocked by
- [ ] https://github.com/getsentry/sentry-java/issues/3527
- [ ] https://github.com/getsentry/sentry-cocoa/issues/4156
- [ ] https://github.com/getsentry/sentry-javascript/issues/12727
``` 