reduce allocations for searching query index (#925)

Iterate the matches list using size/get rather than the
foreach loop which allocates an iterator. This can often
be a hotspot for searching.

spectator-reg-atlas/src/jmh/java/com/netflix/spectator/atlas/ListIteration.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2014-2021 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.netflix.spectator.atlas;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.infra.Blackhole;
+
+import java.util.ArrayList;
+import java.util.List;
+
+@State(Scope.Thread)
+public class ListIteration {
+
+  private List<Integer> data;
+
+  @Setup
+  public void setup() {
+    data = new ArrayList<>(100);
+    for (int i = 0; i < 100; ++i) {
+      data.add(i);
+    }
+  }
+
+  @Benchmark
+  public void forEach(Blackhole bh) {
+    for (Integer i : data) {
+      bh.consume(i);
+    }
+  }
+
+  @Benchmark
+  public void forUsingGet(Blackhole bh) {
+    int n = data.size();
+    for (int i = 0; i < n; ++i) {
+      bh.consume(data.get(i));
+    }
+  }
+}

spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/impl/QueryIndex.java

@@ -396,8 +396,11 @@ private void forEachMatch(Id tags, int i, Consumer<T> consumer) {
               otherChecksCache.put(v, tmp);
             }
           } else {
-            for (QueryIndex<T> idx : otherMatches) {
-              idx.forEachMatch(tags, i + 1, consumer);
+            // Enhanced for loop typically results in iterator being allocated. Using
+            // size/get avoids the allocation and has better throughput.
+            int n = otherMatches.size();
+            for (int p = 0; p < n; ++p) {
+              otherMatches.get(p).forEachMatch(tags, i + 1, consumer);
             }
           }
         } else if (cmp > 0) {