Strawman interface for EventListener

Related to the discussion in square#270, this a class that could be used to add
Zipkin or other traces to OkHttp requests

okhttp/src/main/java/okhttp3/EventListener.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright (C) 2017 Square, 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 okhttp3;
+
+import java.io.IOException;
+
+/**
+ * Observes, modifies, and potentially short-circuits requests going out and the corresponding
+ * responses coming back in. Typically interceptors add, remove, or transform headers on the request
+ * or response.
+ */
+public class EventListener {
+	public static final EventListener NULL_EVENT_LISTENER = new EventListener() {
+	};
+
+	void requestEnqueued(Call call) {
+	}
+
+	void dnsStart(Call call, String domainName) {
+	}
+
+	void dnsEnd(Call call, String domainName, List<InetAddress> inetAddressList, Throwable throwable) {
+	}
+
+	void connectStart(Call call, InetAddress address, int port) {
+	}
+
+	void secureConnectStart(Call call) {
+	}
+
+	void secureConnectEnd(Call call, String cipherSuite, String protocol, Throwable throwable) {
+	}
+
+	void connectEnd(Call call,  InetAddress address, int port, String protocol, Throwable throwable) {
+	}
+
+	void requestHeadersStart(Call call) {
+	}
+
+	void requestHeadersEnd(Call call, Throwable throwable) {
+	}
+
+	void requestBodyStart(Call call) {
+	}
+
+	void requestBodyEnd(Call call, Throwable throwable) {
+	}
+
+	void responseHeadersStart(Call call) {
+	}
+
+	void responseHeadersEnd(Call call, Throwable throwable) {
+	}
+
+	void responseBodyStart(Call call) {
+	}
+
+	void responseBodyEnd(Call call, Throwable throwable) {
+	}
+}