Polish "Add ProducerInterceptor support"

* Add producer interceptor docs (author added in previous commit but were lost during rebase)
* Add test for ordering of multiple configured interceptors

Closes #35

spring-pulsar-docs/src/main/asciidoc/pulsar.adoc

@@ -531,6 +531,36 @@ public void listen(Foo foo) {
 
 On the producer side also, for the Java primitive types, the framework can infer the Schema, but for any other types, you need set that on the `PulsarTemmplate`.
 
+
+=== Intercepting messages
+
+==== Intercept messages on the Producer
+Adding a `ProducerInterceptor`  allows you to intercept and mutate messages received by the producer before being published to the brokers.
+To do so, you can pass a list of interceptors into the `PulsarTemplate` constructor.
+When using multiple interceptors, the order they are applied in will be the order they appear in the list.
+
+If you are using Spring Boot auto-configuration, you can simply specify the interceptors as Beans.
+They will be passed automatically to the `PulsarTemplate`.
+Ordering of the interceptors is achieved by using the `@Order` annotation as seen below.
+
+====
+[source, java]
+----
+@Bean
+@Order(100)
+ProducerInterceptor firstInterceptor() {
+  ...
+}
+
+@Bean
+@Order(200)
+ProducerInterceptor secondInterceptor() {
+  ...
+}
+----
+====
+
+
 ==== Appendix
 The reference documentation has the following appendices:
 

spring-pulsar-spring-boot-autoconfigure/src/test/java/org/springframework/pulsar/autoconfigure/PulsarAutoConfigurationTests.java

@@ -30,6 +30,9 @@ import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.test.context.FilteredClassLoader;
 import org.springframework.boot.test.context.assertj.AssertableApplicationContext;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.annotation.Order;
 import org.springframework.pulsar.annotation.EnablePulsar;
 import org.springframework.pulsar.annotation.PulsarBootstrapConfiguration;
 import org.springframework.pulsar.annotation.PulsarListenerAnnotationBeanPostProcessor;
@@ -167,6 +170,16 @@ class PulsarAutoConfigurationTests {
 						.contains(interceptor)));
 	}
 
+	@Test
+	void customProducerInterceptorsOrderedProperly() {
+		this.contextRunner.withUserConfiguration(InterceptorTestConfiguration.class)
+				.run((context -> assertThat(context).hasNotFailed().getBean(PulsarTemplate.class)
+						.extracting("interceptors")
+						.asInstanceOf(InstanceOfAssertFactories.list(ProducerInterceptor.class))
+						.containsExactly(InterceptorTestConfiguration.interceptorBar,
+								InterceptorTestConfiguration.interceptorFoo)));
+	}
+
 	@Nested
 	class ProducerFactoryAutoConfigurationTests {
 
@@ -207,4 +220,24 @@ class PulsarAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class InterceptorTestConfiguration {
+
+		static ProducerInterceptor interceptorFoo = mock(ProducerInterceptor.class);
+		static ProducerInterceptor interceptorBar = mock(ProducerInterceptor.class);
+
+		@Bean
+		@Order(200)
+		ProducerInterceptor interceptorFoo() {
+			return interceptorFoo;
+		}
+
+		@Bean
+		@Order(100)
+		ProducerInterceptor interceptorBar() {
+			return interceptorBar;
+		}
+
+	}
+
 }