Prior to this commit, the `ServerHttpObservationFilter` was fixed to
re-enable instrumentation for async dispatches. This fix involves using
an AsyncListener to be notified of exchange completion.
This change was incomplete, as this would not work in some cases.
If another filter starts the async mode and initiates an ASYNC dispatch,
before async handling at the controller level, the async listener is not
registered against subsequent async starts.
This commit not only ensures that the async listener registers
against new async starts, but also ensure that the initial creation and
registration only happens during the initial REQUEST dispatch.
Fixes gh-33451
debba6545b