Add benchmarks for DefaultLTT start/stop

Start and stop are called on the critical path. We should have benchmarks for them to evaluate changes that may affect their performance.

benchmarks/benchmarks-core/src/jmh/java/io/micrometer/benchmark/core/DefaultLongTaskTimerBenchmark.java

@@ -0,0 +1,96 @@
+/*
+ * Copyright 2024 VMware, 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
+ *
+ * https://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 io.micrometer.benchmark.core;
+
+import io.micrometer.core.instrument.*;
+import io.micrometer.core.instrument.distribution.DistributionStatisticConfig;
+import io.micrometer.core.instrument.internal.DefaultLongTaskTimer;
+import org.openjdk.jmh.annotations.*;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.RunnerException;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+@State(Scope.Benchmark)
+@OutputTimeUnit(TimeUnit.MICROSECONDS)
+@Fork(1)
+public class DefaultLongTaskTimerBenchmark {
+
+    public static void main(String[] args) throws RunnerException {
+        Options opt = new OptionsBuilder().include(DefaultLongTaskTimerBenchmark.class.getSimpleName()).build();
+
+        new Runner(opt).run();
+    }
+
+    private static final int sampleCount = 10_000;
+
+    private MockClock clock;
+
+    private DefaultLongTaskTimer longTaskTimer;
+
+    private LongTaskTimer.Sample middleSample;
+
+    @Setup(Level.Invocation)
+    public void setup() {
+        clock = new MockClock();
+        longTaskTimer = new DefaultLongTaskTimer(
+                new Meter.Id("ltt", Tags.empty(), TimeUnit.MILLISECONDS.toString().toLowerCase(), null,
+                        Meter.Type.LONG_TASK_TIMER),
+                clock, TimeUnit.MILLISECONDS, DistributionStatisticConfig.DEFAULT, false);
+        // start some samples for benchmarking start/stop with active samples
+        IntStream.range(0, sampleCount).forEach(offset -> {
+            clock.add(offset, TimeUnit.MILLISECONDS);
+            LongTaskTimer.Sample sample = longTaskTimer.start();
+            if (offset == sampleCount / 2)
+                middleSample = sample;
+        });
+    }
+
+    @TearDown(Level.Invocation)
+    public void tearDown() {
+        longTaskTimer = null;
+    }
+
+    @Benchmark
+    @Warmup(iterations = 20)
+    @Measurement(iterations = 200)
+    @BenchmarkMode(Mode.SingleShotTime)
+    public LongTaskTimer.Sample start() {
+        return longTaskTimer.start();
+    }
+
+    @Benchmark
+    @Warmup(iterations = 20)
+    @Measurement(iterations = 200)
+    @BenchmarkMode(Mode.SingleShotTime)
+    public long stop() {
+        return middleSample.stop();
+    }
+
+    @Benchmark
+    @Warmup(iterations = 20)
+    @Measurement(iterations = 200)
+    @BenchmarkMode(Mode.SingleShotTime)
+    public long startAndStop() {
+        start();
+        return stop();
+    }
+
+}