Change parameter type of http.Logger() to interface

[fho]

The http.Logger() interface expects a log.Logger type as parameter.

Instead of expecting log.Logger, define a simple logger interface with the Printf() method and change the parameter to that interface.

That will allow to use other loggers then log.Logger of the stdlib as reporter logger.

[jcchavezs]

We did something similar for the HTTP client, could you at least explain more the use case and why it can't be solved with the current logger (maybe convenience or actual technical challenge).

[basvanbeek]

closing as related to / subset of #37

[fho]

@jcchavezs We use in our applications a different logger then the one from the stdlib.
Currently it is not possible to log messages from zipkin-go to our logger. It's only possible to log messages to the stdlib logger.

I want to use our logger because it e.g. prints messages in JSON-format to stdout.

[basvanbeek]

@fho the use case you describe fits issue #37 which I'll pick up after my holiday. I am not guaranteeing you can directly use your Printf() method but a simple bridge function in between will be possible.