To allow visualization of migrations, track the number flow
immigrations in "port_<N>_immigrations". The "port_<N>_migrations"
retains legacy semantics, but is renamed "port_<N>_emigrations".
Expose the number of events currently undergoing processing
(i.e. pending releases) at a particular port.
Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
c4cf44b677