[Transport] Observer pattern

[Andreas Veithen]

Hi devs,

Already some time ago I had the idea to refactor the existing metrics  
collection code (used by the JMX support) in the transport base  
package to use an observer pattern. More precisely the idea is to  
allow to register any number of observers (or event listeners if you  
prefer) with a transport listener or sender. These observers would  
then be notified whenever something interesting happens in the  
transport (message received, error, etc.). Basically the refactoring  
would be achieved by extracting an interface from MetricsCollector and  
change the 1:1 relationship between transport and observer (metrics  
collector) into a 1:n relationship. The existing MetricsCollector  
would then become one particular observer.

I'm primarily interested in this change because I'm facing a  
particular problem in the transport unit tests: the test cases have no  
way to detect a reception error in the transport, except for waiting  
for the timeout. This could be solved using the observer pattern  
described above. Probably this particular use case on its own wouldn't  
justify the change, but there will probably be other interesting use  
cases.

WDYT?

Andreas