The otel trace tests asserted that a streamed call produces exactly one gen-AI
span. The proxy opens one gen-AI span per upstream attempt, so a call the
router retried carries an error span for every failed attempt beside the one
that answered, and the assertion fails on a request that succeeded.
Select the served attempt instead: drop spans whose otel.status_code is ERROR,
require exactly one survivor, and run the TTFT and streaming-flag assertions
against it. That keeps what these assertions exist for, a split trace or a
stream logged as two served spans, while tolerating a retry.
Only the failed attempt lacks TTFT, so the old code also had a second failure
mode: when the first span happened to be the error one, the test reported the
attribute as missing rather than as belonging to a different attempt.
test_span_selection.py covers the selection itself against Jaeger-shaped
payloads and carries no e2e marker, since reproducing a first-attempt failure
live is not something a test can arrange.
