Autodetect consumer producer names

[sigilioso]

Closes #159

This PR introduces support to detect automatically consumer/producer client.id. As agreed in this internal link, we made the name field optional for consumers and producers and introduced the default alias, so:

    PRODUCERS: '[{"host": "localhost", "port": 24, "username": "me", "password": "secret"}]' # get metrics from all producers detected in localhost:24

    PRODUCERS: '[{}]' # Get metrics from all producers detected in DEFAULT_JMX_HOST:DEFAULT_JMX_PORT
    
    PRODUCERS: 'default' # It is an alias of '[{}]' (will gather metrics from all producers in default host:port)

    PRODUCERS: '[]' # (default value), no Java metrics from producers are got

    PRODUCERS: '[{"name": "myProducer"}]' # get metrics only the producer whose client.id is "myProducer" in DEFAULT_JMX_HOST:DEFAULT_JMX_PORT

[alvarocabanas]

LGTM! Nice job!

[alvarocabanas]

I left a comment.

[alvarocabanas]

LGTM, like this we are behaving like we used to and making the fallback to app-info