stats_cb support

[fkulakov]

Hello. I wants to support stats_cb config field first of all for my own needs but for overall system flexibility too.

[Steveb-p]

While configuration-wise it should work, it will probably fail due to #749. Specifically because librdkafka::poll is never called internally, which also causes any callbacks to not fire.

I'll look into fixing the abovementioned issue tonight, if that's alright for you @fkulakov.

See: https://github.com/edenhill/librdkafka/wiki/Consumer-lag-monitoring

librdkafka automatically monitors consumer lag for RD_KAFKA_CONSUMER handles and the information is available in the statistics JSON object as emitted by stats_cb. A statistics callback is set up with rd_kafka_conf_set_stats_cb() and the callback is called at the configured interval (statistics.interval.ms) from an rd_kafka_poll() call.

[Steveb-p]

Totally forgot :)
Thanks for contributing @fkulakov