Cache Tracer Host Entry to reduce allocation in RemoteHostnameAdder (#10968)

Caching host entry to reduce allocation

spotless

Merge branch 'master' into dougqh/cache-tracer-host

Merge branch 'master' into dougqh/cache-tracer-host

Flipped equals order

hostname is type String while cachedHostEntry.objectValue() returns Object

By calling equals on hostname, the equals call can be statically devirtualized
Previously, the call would have used type profile based devirtualization, but that incurs an extra type check that isn't really needed

Admittedly, there will still be a type check in string equals

Merge branch 'master' into dougqh/cache-tracer-host

Merge branch 'master' into dougqh/cache-tracer-host

Merge branch 'master' into dougqh/cache-tracer-host

Merge branch 'master' into dougqh/cache-tracer-host

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: dougqh

dd-trace-core/src/main/java/datadog/trace/core/tagprocessor/RemoteHostnameAdder.java

@@ -9,15 +9,32 @@
 public final class RemoteHostnameAdder extends TagsPostProcessor {
   private final Supplier<String> hostnameSupplier;
 
+  private TagMap.Entry cachedHostEntry = null;
+
   public RemoteHostnameAdder(Supplier<String> hostnameSupplier) {
     this.hostnameSupplier = hostnameSupplier;
   }
 
   @Override
   public void processTags(
       TagMap unsafeTags, DDSpanContext spanContext, AppendableSpanLinks spanLinks) {
-    if (spanContext.getSpanId() == spanContext.getRootSpanId()) {
-      unsafeTags.put(DDTags.TRACER_HOST, hostnameSupplier.get());
+    if (spanContext.getSpanId() != spanContext.getRootSpanId()) {
+      return;
+    }
+
+    String hostname = hostnameSupplier.get();
+    if (hostname == null) {
+      return;
     }
+
+    TagMap.Entry cachedHostEntry = this.cachedHostEntry;
+    if (cachedHostEntry != null && hostname.equals(cachedHostEntry.objectValue())) {
+      unsafeTags.set(cachedHostEntry);
+      return;
+    }
+
+    TagMap.Entry newEntry = TagMap.Entry.create(DDTags.TRACER_HOST, hostname);
+    unsafeTags.set(newEntry);
+    this.cachedHostEntry = newEntry;
   }
 }