docs: add resource-timing-event.md and update browser-observability-model.md

[overbalance]

Summary

Create a docs/resource-timing-event.md document (following the pattern of docs/navigation-event.md) that defines the browser.resource_timing event and its attribute mappings. Update docs/browser-observability-model.md to link to the new doc and reference the latest semconv status.

This is based on the unified HTTP client network timing conventions decided in semantic-conventions#3385.

Tasks

1. Create docs/resource-timing-event.md with purpose, use cases, and full attribute mapping tables
2. Update docs/browser-observability-model.md:
   • Link the browser.resource_timing event name to the new doc
   • Update Semantic Conventions Status to reference semantic-conventions#3385

Key decisions from semconv#3385

• Signal type: Event (not metric, not span attributes)
• Event name: browser.resource_timing
• Delta time model: http.call.start_time is absolute (ms since epoch); all others are deltas from it
• Browser API values of 0 mean the phase did not occur -- omit the attribute
• Browser natively provides deltas relative to timeOrigin; instrumentation converts to deltas relative to http.call.start_time
• Events work better for browser because timing data arrives asynchronously and the API covers all resource types (CSS, JS, images, etc.), not just fetch/XHR

Event metadata

• Timestamp: http.call.start_time (when the fetch started)
• ObservedTimestamp: when the event is emitted
• Context: context of the original HTTP span (if one exists) for correlation

Timing attribute mapping

Browser API field	Proposed attribute	Requirement	Notes
fetchStart	http.call.start_time	Required	Absolute timestamp (ms since epoch)
responseEnd	http.call.end_time	Required	Delta from start_time
redirectStart	http.redirect.start_time	Opt-In	Omit if 0 (no redirect). Browser only.
redirectEnd	http.redirect.end_time	Opt-In	Omit if 0. Browser only.
domainLookupStart	http.dns.start_time	Opt-In	Omit if 0 (reused connection)
domainLookupEnd	http.dns.end_time	Opt-In
connectStart	http.connect.start_time	Opt-In
connectEnd	http.connect.end_time	Opt-In	Also implicitly TLS end
secureConnectionStart	http.secure_connect.start_time	Opt-In	HTTPS only, omit if 0
requestStart	http.request.headers.start_time	Opt-In	Browser exposes only a single request start
responseStart	http.response.headers.start_time	Opt-In	First byte received (TTFB)
workerStart	http.worker.start_time	Opt-In	Omit if 0 (no service worker). Browser only.

Browser does not expose: http.secure_connect.end_time, http.request.headers.end_time, http.request.body.*, http.response.headers.end_time, http.response.body.*

Additional resource attributes

Browser API field	Proposed attribute	Notes
responseStatus	http.response.status_code
encodedBodySize	http.response.body.size
transferSize	http.response.size
decodedBodySize	TBD	May need separate attribute
contentType	http.response.header.content-type	Value from Resource Timing API
nextHopProtocol	network.protocol.name + network.protocol.version	Needs parsing
initiatorType	TBD	Browser-specific, likely separate namespace
deliveryType	TBD	Browser-specific, likely separate namespace
renderBlockingStatus	TBD	Browser-specific, likely separate namespace
serverTiming	TBD	Possibly http.response.header.server-timing

Optional: copy of original HTTP span attributes

A configurable option to include original HTTP span attributes on the event for backends that cannot yet correlate signals:

Attribute	Type	Example
url.full	string	https://api.example.com/users
http.request.method	string	GET
http.response.status_code	int	200
server.address	string	example.com
server.port	int	443
network.protocol.version	string	2

Open questions

• Should timing attributes be modeled as complex attributes? (raised by @breedx-splk)
• All vs curated list of original HTTP span attributes? (raised by @bidetofevil)
• Attribute naming for dns/tls/worker -- thompson-tomo proposed dns.question.*, tls.connect.*, browser.web_worker.* instead of http.dns.*, http.secure_connect.*, http.worker.*