Add support for the rdkafka 'stats_cb' config option in JournalClient
this options allows to define a callback which will be called once every 'statistics.interval.ms' ms by rdkafka with a bunch of statistics (as a json string).
See https://github.com/edenhill/librdkafka/blob/master/STATISTICS.md
This allows to define this callback as a string of the form:
"path.to.module:function"
Depends on !212 (closed)
Test Plan
WIP because needs tests
Migrated from D6884 (view on Phabricator)