Investigate traces with missing root spans

[Lms24]

Some traces appear to be missing the trace root span, and there also seems to be a large amount of <unknown> spans. This may be related to streaming, but the behavior was not consistently reproducible.

[linear-code]

JS-2870

[Lms24]

Going to close this for the moment because the "Missing Trace Root" doesn't seem to be SDK-related after all. Nevertheless, I opened #21862 which helps ensuring we flush pending spans from the buffer. This could partially explain cases where a pageload span was missing from the trace but then again, we couldn't yet reproduce/proof that the SDK is at fault.

Other "Missing Trace Root" cases:

• Bug: the trace root detection logic seems to depend on the selected time frame of the trace explorer. Not an SDK issue.
• Our fault: we didn't correctly continue a parent sampling decision for SSR traces, leading to cases where we'd start a new pageload span tree for a negatively sampled http.server parent span tree but keep referencing the parent span. This was a sentry SDK misconfiguration fixed via fix(sdk): Inherit parent sampling decision instead of overriding it sentry#118736