There is an edge case, if you use `forward/4` and use Plug.ErrorHandler,
then when an exception reaches the outer router, then Plug.send_resp
will be called, triggering `[:phoenix, :endpoint, :stop]`, and the span
will be gone by the time the outer router gets the exception. This
causes this telemetry handler to crash and be detached.
Sequence of events:
- [:phoenix, :endpoint, :start]
- [:phoenix, :router_dispatch, :exception] (inner router)
- [:phoenix, :endpoint, :stop]
- [:phoenix, :router_dispatch, :exception] (outer router) ** here there is no span, crashes
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