feat: pub ProducerClient for instrumentation hook

[domodwyer]

Marks the ProducerClient as pub, so external code can inject instrumentation over the kafka client via decorators when constructing the BatchProducer.

This allows:

• Recording the latency of the low-level produce call independent of the aggregation logic (just the serialisation / protocol overhead / network I/O / etc)
• Inspecting the Record payloads on the producer side (somewhat related to https://github.com/influxdata/influxdb_iox/issues/5117)
• Emitting metrics for produce() errors, etc

This is not a breaking API change.

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• feat: pub ProducerClient for instrumentation hook (9bf370a)

  This commit changes the visibility of the ProducerClient abstraction and the
  associated BatchProducer constructor method (used internally to swap out the
  real client impl with a mock for testing purposes.)
  
  This allows external code to optionally "hook" into the lower-level Kafka
  client calls, in order to inspect the Records being pushed to Kafka /
  instrument & evaluate produce errors / measure produce latency / etc.