Add topic and partition to acknowledged_message event

[maeve]

The standalone producer includes the topic and partition in the instrumentation payload for produce_async events. However, when the broker acknowledges message delivery, the payload for the acknowledged_message event only contains the partition and offset, but not the topic. This is of limited utility, since multiple topics are likely to have the same partition numbers.

The rdkafka delivery report passed into the callback already includes the topic_name, so I added that information to the event payload. I also updated Racecar::Datadog::ProducerSubscriber to add tags for the topic and partition to the producer.ack.messages metric reported to StatsD.