fix: Bad function type for beforeSendSpan

[zhiyan114]

Fixed a type issue where beforeSendSpan cannot return null despite the docs says it's used to drop spans.

[Lms24]

Hey @zhiyan114 thanks for opening this PR! Unfortunately, beforeSendSpan really can't return null since version 9 of the SDK. Ever since this version, we cannot let spans being dropped retroactively because doing so would risk span metrics extrapolation being off.

I have two questions for you:

• which spans did you want to drop (if any)?
• can you link to the docs where it says that the beforeSendSpan callback can return null? We need to adjust the docs.
  • update: I think I found it. Can you confirm that this is the place?
  • Opened fix(js): Fix incorrect beforeSendSpan callback type sentry-docs#13906 to fix the type signature

I'm going to close this PR because unfortunately, the type change does not apply anymore. Still, thanks a lot for contributing!