title: where does confluent s3 sink put the key?
body: <p>I setup a confluent s3 sink connect, it stores .avro files in s3.<br>
  I dump those files, and find out that they are just the message itself, I don't know where can I find the message key, any idea?</p>
  
  <p>The config is like:</p>
  
  <pre><code>{
      "name": "s3-sink-test",
      "config": {
          "connector.class": "io.confluent.connect.s3.S3SinkConnector",
          "tasks.max": "1",
          "topics": "book",
          "s3.region": "eu-central-1",
          "s3.bucket.name": "kafka",
          "s3.part.size": "5242880",
          "storage.class": "io.confluent.connect.s3.storage.S3Storage",
          "format.class": "io.confluent.connect.s3.format.avro.AvroFormat",
          "schema.generator.class": "io.confluent.connect.storage.hive.schema.DefaultSchemaGenerator",
          "partitioner.class": "io.confluent.connect.storage.partitioner.TimeBasedPartitioner",
          "path.format": "'year'=YYYY/'month'=MM/'day'=dd/'hour'=HH",
          "locale": "US",
          "timezone": "UTC",
          "partition.duration.ms": "3600000",
          "timestamp.extractor": "RecordField",
          "timestamp.field": "local_timestamp",
          "flush.size": "2",
          "schema.compatibility": "NONE"
      }
  }
  </code></pre>