opti test and add javadoc

Author: hqbfzwang

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ProducerImpl.java

@@ -81,10 +81,7 @@
 import org.apache.pulsar.client.impl.conf.ProducerConfigurationData;
 import org.apache.pulsar.client.impl.crypto.MessageCryptoBc;
 import org.apache.pulsar.client.impl.metrics.InstrumentProvider;
-import org.apache.pulsar.client.impl.metrics.LatencyHistogram;
 import org.apache.pulsar.client.impl.metrics.ProducerMetrics;
-import org.apache.pulsar.client.impl.metrics.Unit;
-import org.apache.pulsar.client.impl.metrics.UpDownCounter;
 import org.apache.pulsar.client.impl.schema.AutoProduceBytesSchema;
 import org.apache.pulsar.client.impl.schema.JSONSchema;
 import org.apache.pulsar.client.impl.schema.SchemaUtils;