1.2.0: upstream request-ID correlation
End-to-end debugging unlock that lets operators jump from a slow
beacon to the corresponding edge log, APM trace, or load-balancer
log entry by correlating on the upstream-stack request ID.
UpstreamIdSubscriber reads a configured list of request headers
(typically cf-ray for Cloudflare, x-amzn-trace-id for AWS,
x-datadog-trace-id, x-cloud-trace-context for Google Cloud, or
generic x-request-id) on the page request and stamps the first
non-empty value into a cwv-upstream-id Server-Timing entry on
the response. UpstreamIdCollector reads the entry from the
beacon's server_timing array and writes context_data.upstream_id.
Operators query JSON_EXTRACT(context_data, '$.upstream_id') to
correlate slow beacons with their upstream-stack log entries.
Empty by default (upstream_id_headers: {}). Operators opt in by
listing the headers their stack provides. Adds one config key
via cwv_update_10034. No schema changes. No contract changes.