✨ support asynchronous beforeSend callbacks

[BenoitZugmeyer]

Motivation

Allow RUM and Logs beforeSend callbacks to perform asynchronous enrichment or filtering before an event is sent—for example, reading a fetch response from the resource event domain context.

Changes

• Support Promise<boolean> results from RUM and Logs beforeSend callbacks while preserving synchronous callback behavior.
• Centralize callback execution, error handling, and RUM field-modification enforcement in Browser Core.
• Add unit coverage and an E2E scenario that enriches a RUM resource event from its fetch response.

Test instructions

1. In the sandbox, initialize RUM with an async beforeSend callback.
2. For fetch resource events, read context.response.text() and add the result to the event context.
3. Make a fetch request and verify that the resulting RUM resource event contains the added context value.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 74.36%
• Overall Coverage: 77.24% (+0.04%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 24b825a | Docs | View more details | Give us feedback!}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	180.54 KiB	180.81 KiB	+274 B	+0.15%	✅
Rum Profiler	8.43 KiB	8.43 KiB	0 B	0.00%	✅
Rum Recorder	21.12 KiB	21.12 KiB	0 B	0.00%	✅
Logs	57.04 KiB	57.17 KiB	+141 B	+0.24%	✅
Rum Salesforce	N/A	138.75 KiB	N/A	N/A	N/A
Rum Slim	138.49 KiB	138.75 KiB	+267 B	+0.19%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	200.38 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.43 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	3.72 KiB	N/A	N/A	N/A

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[mormubis]

trackEventCounts also subscribes to RUM_EVENT_COLLECTED and its counters end up in view.action.count, view.error.count, etc. through processViewUpdate. With an async callback the child event is counted when the promise resolves instead of when it's collected, so the final update triggered in view.end() can ship with lower counts. startRumEventBridge has the same gap since it never goes through createBatchDispatcher. Should this live in the assembly so every RUM_EVENT_COLLECTED consumer gets it?

[mormubis]

The rate limit check now runs after the promise resolves, and I think that breaks allowNextEvent in createEventRateLimiter. It sets the flag, calls onLimitReached, then resets it in a finally, so it only works if the agent error reaches isLimitReached() in the same tick. With an async callback the promise is still pending at that point, so the "Reached max number of errors by minute" event gets dropped too. Logs isn't affected thanks to the log.origin !== ErrorSource.AGENT guard, should we add the same one here?