Replaces user-defined type EmptyNull with the
nullable defined by the OpenAPI specification.

The Finalize collector method is called after all the waits and the
timeouts for finishing the ongoing connection attempts have finished.

improve openapi docs, documentaing returned 
null instead of empty array

The following extended metrics information are exposed through
the debug API:
- last seen timestamp - updated every time when the peer
connects/disconnects
- connection total duration - sums of the duration of the sessions
  (in seconds)
- session connection retry count - number of attempts to create
  a new connection
- session connection duration - duration (in seconds) of last or
  current (if in progress) session
- session connection direction - specifies whether the connection is
  outbound or inbound