Tracing: Always end write stream on shutdown in dev (#70811)
While we don’t want to end the write stream in other situations in dev,
we _do_ want to do this when shutting down the web server.
This previously resulted in some spans, including the top-level
`next-dev` span, not always being written to disk as the process would
exit before it was flushed.
Test Plan: Ensured `next-dev` span is written to trace on devserver
shutdown
