⚗️ Track Server-Sent Events counts on RUM resources

[samcolmanetti]

Context

A text/event-stream fetch is opaque in RUM today: no visibility into how many events streamed, of which types, or when the stream went quiet. This adds a compact resource.sse object to the resource event, behind the sse_event_counts experimental flag. Wire-schema (rum-events-format) update is a tracked follow-up; ingestion already accepts the field.

Note: There's also work underway to support WebSockets (design doc). There may be room to consolidate both into a generalized "event stream" abstraction down the line. The key difference: SSE already appears in RUM as a normal Resource (type: fetch) and only lacks this metadata, whereas I don't believe WebSocket connections show up as RUM resources today.

Changes

• Count fetch SSE events incrementally without retaining the body
• Release page activity at headers; read stream in background
• Deliver counts out of band, attach at resource assembly
• Bound reads with byte and time caps

Test Plan

• Unit: SSE parser + counter (LF/CRLF/bare-CR, chunk splits, caps, _other_, id truncation)
• Unit: resource enrichment attaches out-of-band counts; request/resource specs pass
• typecheck and lint clean
• Manual: /sse dev endpoint yields resource.sse; SSE-triggered click closes without hitting the 10s max
• Staging

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Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	172.40 KiB	175.00 KiB	+2.59 KiB	+1.51%	✅
Rum Profiler	7.88 KiB	7.88 KiB	+1 B	+0.01%	✅
Rum Recorder	21.22 KiB	21.22 KiB	0 B	0.00%	✅
Logs	54.82 KiB	56.99 KiB	+2.17 KiB	+3.97%	✅
Rum Slim	130.14 KiB	132.63 KiB	+2.49 KiB	+1.91%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅

[datadog-prod-us1-6]

 

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 1 Pipeline job failed

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ℹ️ Info

No other issues found (see more)

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 57.05%
• Overall Coverage: 76.44% (-0.41%)

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