Merge PR #396: [WIP] Update kafka-exporter module to follow style guide

Author: trask

kafka-exporter/src/main/java/io/opentelemetry/contrib/kafka/KafkaSpanExporter.java

@@ -26,7 +26,7 @@
 
 @ThreadSafe
 @SuppressWarnings("FutureReturnValueIgnored")
-public class KafkaSpanExporter implements SpanExporter {
+public final class KafkaSpanExporter implements SpanExporter {
   private static final Logger logger = LoggerFactory.getLogger(KafkaSpanExporter.class);
   private final String topicName;
   private final Producer<String, Collection<SpanData>> producer;

kafka-exporter/src/main/java/io/opentelemetry/contrib/kafka/KafkaSpanExporterBuilder.java

@@ -20,7 +20,7 @@
 import org.apache.kafka.clients.producer.Producer;
 import org.apache.kafka.common.serialization.Serializer;
 
-public class KafkaSpanExporterBuilder {
+public final class KafkaSpanExporterBuilder {
   private static final long DEFAULT_TIMEOUT_IN_SECONDS = 5L;
   private String topicName;
   private Producer<String, Collection<SpanData>> producer;
@@ -67,7 +67,7 @@ public KafkaSpanExporter build() {
     return new KafkaSpanExporter(topicName, producer, executorService, timeoutInSeconds);
   }
 
-  public static class ProducerBuilder {
+  public static final class ProducerBuilder {
     private Map<String, Object> config;
     private Serializer<String> keySerializer;
     private Serializer<Collection<SpanData>> valueSerializer;

kafka-exporter/src/main/java/io/opentelemetry/contrib/kafka/SpanDataDeserializer.java

@@ -11,7 +11,7 @@
 import org.apache.kafka.common.errors.SerializationException;
 import org.apache.kafka.common.serialization.Deserializer;
 
-public class SpanDataDeserializer implements Deserializer<ExportTraceServiceRequest> {
+public final class SpanDataDeserializer implements Deserializer<ExportTraceServiceRequest> {
   @SuppressWarnings("NullAway")
   @Override
   public ExportTraceServiceRequest deserialize(String topic, byte[] data) {

kafka-exporter/src/main/java/io/opentelemetry/contrib/kafka/SpanDataSerializer.java

@@ -20,7 +20,7 @@
 import org.apache.kafka.common.errors.SerializationException;
 import org.apache.kafka.common.serialization.Serializer;
 
-public class SpanDataSerializer implements Serializer<Collection<SpanData>> {
+public final class SpanDataSerializer implements Serializer<Collection<SpanData>> {
   @Override
   public byte[] serialize(String topic, Collection<SpanData> data) {
     if (Objects.isNull(data)) {