feat: Add before_send_transaction

[JoshuaMoelans]

Fixes #1214 (and related to #864 (comment))

ToDo

• add option
• call inside transaction event preparation
• add to example
• answer todo questions in-code
• add tests
• changelog
• docs feat(native): update transaction docs sentry-docs#13715

questions (resolved)

• Why do we have invoke_before_send passed into prepare_event (see code), and why is it true for sentry_core occurrences (1, 2), but passed as !options->on_crash_func for breakpad?

  For crashes, we only before_send if there is no on_crash hook.

• Where do we callback? Before or after applying scope?

  As late as possible, to give the user maximal context.

• (void* hint) parameter (which is unused for the other callbacks, but exists afaict), see here

  Not relevant for before_send_transaction.

• Do we want to add before_transaction_data to modify transactions before sending? see here

  Yes.

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[supervacuus]

Why do we have invoke_before_send passed into prepare_event (see code), and why is it true for sentry_core occurrences (1, 2), but passed as !options->on_crash_func for breakpad?

Because this function is used for regular event capture, and during the capture of crashes from the handler. We want to invoke the before_send hook with regular events, because that is what we use it for. For crashes, we only run the before_send hook if no on_crash hook has been defined. This is documented in the respective hook docs.

Where do we callback? Before or after applying scope?

As late as possible: after applying the scope. What would your argument be against that?

(void* hint) parameter (which is unused for the other callbacks, but exists afaict), see here

The hint parameter isn't really applicable to the Native SDK, but we could pass in the ucontext or exception_ptrs if it is used in the crash context as we do for the on_crash hook (but in an untyped fashion). I don't think it is relevant to the before_send_transaction.

Do we want to add before_transaction_data to modify transactions before sending? see here

I think so, but similar to the concrete place of calling the callback I would compare with other SDKs.

[supervacuus]

Looks great. Just wonder about the parameter naming and whether we should use the PR to make its use consistent.