Configure Reactor instrumentation to wrap onLastOperator or onEachOperator

with this feature we would allow user to pick whether they instrument on each or on last operator.

fixes gh-1478

benchmarks/src/main/java/org/springframework/cloud/sleuth/benchmarks/jmh/benchmarks/SpringWebFluxOnLastBenchmark.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2013-2019 the original author or authors.
+ *
+ * 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 org.springframework.cloud.sleuth.benchmarks.jmh.benchmarks;
+
+public class SpringWebFluxOnLastBenchmark extends SpringWebFluxBenchmarks {
+
+	@Override
+	protected String[] runArgs() {
+		return new String[] { "--spring.jmx.enabled=false",
+				"--spring.application.name=defaultTraceContextWithOnLastOperator",
+				"--spring.sleuth.enabled=true",
+				"--spring.sleuth.reactor.on-each-operator=false" };
+	}
+
+}