MINOR: connection: Add samples to retrieve info on streams for a connection

MINOR: connection: Add samples to retrieve info on streams for a connection

Thanks to the previous fix, it is now possible to get the number of opened
streams for a connection and the negociated limit. Here, corresponding
sample feches are added, in fc_ and bc_ scopes.

On frontend side, the limit of streams is imposed by HAProxy. But on the
backend side, the limit is defined by the server. it may be useful for
debugging purpose because it may explain slow-downs on some processing.