Add comments and remove unneeded code.

dd-trace-ot/src/main/java/datadog/opentracing/DDTracer.java

@@ -564,14 +564,7 @@ public class DDTracer implements io.opentracing.Tracer, Closeable, datadog.trace
 
         // Get header tags whether propagating or not.
         if (parentContext instanceof TagContext) {
-          final TagContext tagContext = (TagContext) parentContext;
-
-          if (tags.isEmpty() && !tagContext.getTags().isEmpty()) {
-            tags = new HashMap<>();
-          }
-          if (!tagContext.getTags().isEmpty()) {
-            tags.putAll(tagContext.getTags());
-          }
+          tags.putAll(((TagContext) parentContext).getTags());
         }
         tags.put(Config.RUNTIME_ID_TAG, runtimeId);
 

dd-trace-ot/src/main/java/datadog/opentracing/propagation/ExtractedContext.java

@@ -3,6 +3,9 @@ package datadog.opentracing.propagation;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+/**
+ * Propagated data resulting from calling tracer.extract with header data from an incoming request.
+ */
 public class ExtractedContext extends TagContext {
   private final String traceId;
   private final String spanId;

dd-trace-ot/src/main/java/datadog/opentracing/propagation/TagContext.java

@@ -4,6 +4,10 @@ import io.opentracing.SpanContext;
 import java.util.Collections;
 import java.util.Map;
 
+/**
+ * When calling extract, we allow for grabbing other configured headers as tags. Those tags are
+ * returned here even if the rest of the request would have returned null.
+ */
 public class TagContext implements SpanContext {
   private final Map<String, String> tags;