Stop allocating BytesRefBuilder for each StreamOutput

We can use a thread-local here, no need to allocate a fresh instance
for each output when they mostly go unused anyway.

Author: original-brownbear

server/src/main/java/org/elasticsearch/common/io/stream/StreamOutput.java

@@ -370,11 +370,12 @@ public void writeOptionalText(@Nullable Text text) throws IOException {
         }
     }
 
-    private final BytesRefBuilder spare = new BytesRefBuilder();
+    private static final ThreadLocal<BytesRefBuilder> spareBytesRefBuilder = ThreadLocal.withInitial(BytesRefBuilder::new);
 
     public void writeText(Text text) throws IOException {
         if (text.hasBytes() == false) {
             final String string = text.string();
+            var spare = spareBytesRefBuilder.get();
             spare.copyChars(string);
             writeInt(spare.length());
             write(spare.bytes(), 0, spare.length());