Add EmbeddedServerPortFileWriter

Add a EmbeddedServerPortFileWriter which can be used to write server
port information to a file.

Fixes gh-1275
Closes gh-1491

spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -856,30 +856,38 @@ if needed.
 
 [[production-ready-process-monitoring]]
 == Process monitoring
-In Spring Boot Actuator you can find `ApplicationPidFileWriter` which creates file
-containing application PID (by default in application directory and file name is
-`application.pid`). It's not activated by default, but you can do it in two simple
-ways described below.
+In Spring Boot Actuator you can find a couple of classes to create files that are useful
+for process monitoring:
+
+* `ApplicationPidFileWriter` creates a file containing the application PID (by default in
+  the application directory with the file name `application.pid`).
+* `EmbeddedServerPortFileWriter` creates a file (or files) containing the ports of the
+  embedded server (by default in the application directory with the file name
+  `application.port`).
+
+These writers are not activated by default, but you can enable them in one of the ways
+described below.
 
 
 
 [[production-ready-process-monitoring-configuration]]
 === Extend configuration
-In `META-INF/spring.factories` file you have to activate the listener:
+In `META-INF/spring.factories` file you have to activate the listener(s):
 
 [indent=0]
 ----
 	org.springframework.context.ApplicationListener=\
-	org.springframework.boot.actuate.system.ApplicationPidFileWriter
+	org.springframework.boot.actuate.system.ApplicationPidFileWriter,
+	org.springframework.boot.actuate.system.EmbeddedServerPortFileWriter
 ----
 
 
 
 [[production-ready-process-monitoring-programmatically]]
 === Programmatically
-You can also activate this listener by invoking `SpringApplication.addListeners(...)`
-method and passing `ApplicationPidFileWriter` object. You can also customize file name
-and path through constructor.
+You can also activate a listener by invoking the `SpringApplication.addListeners(...)`
+method and passing the appropriate `Writer` object. This method also allows you to
+customize file name and path via the `Writer` constructor.