Prior to this commit, the `ServerHttpObservationFilter` would support
async dispatches and would do the following:

1. start the observation
2. call the filter chain
3. if async has started, do nothing
4. if not in async mode, stop the observation

This behavior would effectively rely on Async implementations to
complete and dispatch the request back to the container for an async
dispatch. This is what Spring web frameworks do and guarantee.

Some implementations complete the async request but do not dispatch
back; as a result, observations could leak as they are never stopped.

This commit changes the support of async requests. The filter now
opts-out of async dispatches - the filter will not be called for those
anymore. Instead, if the application started async mode during the
initial container dispatch, the filter will register an AsyncListener to
be notified of the outcome of the async handling.

Fixes gh-32986