Changing traceSource to volatile rather than an AtomicInteger

This avoids allocated an AtomicInteger for every PTagsFactory.
The concurrency needs are met through an AtomicFieldUpdater.

In benchmarks, the AtomicFieldUpdater appears the same or better with lower memory overhead.

Author: dougqh

dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/PTagsFactory.java

@@ -18,7 +18,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 import javax.annotation.Nonnull;
 
 public class PTagsFactory implements PropagationTags.Factory {
@@ -84,7 +84,10 @@ static class PTags extends PropagationTags {
     // extracted decision maker tag for easier updates
     private volatile TagValue decisionMakerTagValue;
 
-    private final AtomicInteger traceSource;
+    private static final AtomicIntegerFieldUpdater<PTags> AFU_TRACE_SOURCE =
+        AtomicIntegerFieldUpdater.newUpdater(PTags.class, "traceSource");
+
+    private volatile int traceSource;
     private volatile String debugPropagation;
 
     // xDatadogTagsSize of the tagPairs, does not include the decision maker tag
@@ -152,7 +155,7 @@ public PTags(
       this.tagPairs = tagPairs;
       this.canChangeDecisionMaker = decisionMakerTagValue == null;
       this.decisionMakerTagValue = decisionMakerTagValue;
-      this.traceSource = new AtomicInteger(traceSource);
+      this.traceSource = traceSource;
       this.samplingPriority = samplingPriority;
       this.origin = origin;
       this.lastParentId = lastParentId;
@@ -230,7 +233,8 @@ private void doUpdateTraceSamplingPriority(int samplingPriority, int samplingMec
 
     @Override
     public void addTraceSource(final int product) {
-      traceSource.updateAndGet(
+      AFU_TRACE_SOURCE.updateAndGet(
+          this,
           currentValue -> {
             // If the product is already marked, return the same value (no change)
             if (ProductTraceSource.isProductMarked(currentValue, product)) {
@@ -248,7 +252,7 @@ public void addTraceSource(final int product) {
 
     @Override
     public int getTraceSource() {
-      return traceSource.get();
+      return traceSource;
     }
 
     @Override
@@ -386,7 +390,7 @@ int getXDatadogTagsSize() {
         size = PTagsCodec.calcXDatadogTagsSize(getTagPairs());
         size = PTagsCodec.calcXDatadogTagsSize(size, DECISION_MAKER_TAG, decisionMakerTagValue);
         size = PTagsCodec.calcXDatadogTagsSize(size, TRACE_ID_TAG, traceIdHighOrderBitsHexTagValue);
-        int currentProductTraceSource = traceSource.get();
+        int currentProductTraceSource = traceSource;
         if (currentProductTraceSource != ProductTraceSource.UNSET) {
           size =
               PTagsCodec.calcXDatadogTagsSize(