Added Jetty reference doc (#4584)

docs/antora.yml

@@ -18,3 +18,4 @@ asciidoc:
     include-java: 'example$docs-src/test/java/io/micrometer/docs'
     include-resources: 'example$docs-src/test/resources'
     include-core-test-java: 'example$core-test'
+    include-micrometer-test-java: 'example$micrometer-test'

docs/modules/ROOT/examples/micrometer-test

@@ -0,0 +1 @@
+../../../../micrometer-test/src/test/java

docs/modules/ROOT/pages/reference/jetty.adoc

@@ -1,3 +1,6 @@
+[[overview]]
+= Eclipse Jetty and Jersey Instrumentation
+
 Micrometer supports binding metrics to `Jetty` through `Connection.Listener`.
 
 You can collect metrics from `Jetty` by adding `JettyConnectionMetrics` as follows:
@@ -23,4 +26,16 @@ resourceConfig.register(new MetricsApplicationEventListener(
                 "http.server.requests",
                 true));
 ServletContainer servletContainer = new ServletContainer(resourceConfig);
-----
+----
+
+[[overview-observation]]
+== Eclipse Observation Jersey Instrumentation
+
+Below you can find an example of how to instrument Jersey with https://docs.micrometer.io/micrometer/reference/observation.html[Micrometer Observation]. That means that depending on your Observation Handler configuration you instrument once, and can have multiple benefits out of it (e.g. metrics, distributed tracing).
+
+[source,java,subs=+attributes]
+-----
+// Setting up instrumentation
+include::{include-micrometer-test-java}/io/micrometer/core/instrument/JerseyServerTimingInstrumentationVerificationTests.java[tags=setup, indent=0]
+include::{include-micrometer-test-java}/io/micrometer/core/instrument/JerseyServerTimingInstrumentationVerificationTests.java[tags=setup_2, indent=0]
+-----

micrometer-test/src/test/java/io/micrometer/core/instrument/JerseyServerTimingInstrumentationVerificationTests.java

@@ -44,7 +44,11 @@ protected URI startInstrumentedWithMetricsServer() throws Exception {
     @Nullable
     @Override
     protected URI startInstrumentedWithObservationsServer() throws Exception {
-        jerseyTest = jerseyWithListener(new ObservationApplicationEventListener(getObservationRegistry(), timerName()));
+        jerseyTest = jerseyWithListener(
+                // tag::setup[]
+                new ObservationApplicationEventListener(getObservationRegistry(), timerName())
+        // end::setup[]
+        );
         return setupUri(jerseyTest);
     }
 
@@ -57,8 +61,10 @@ private JerseyTest jerseyWithListener(Object listener) {
         return new JerseyTest() {
             @Override
             protected Application configure() {
+                // tag::setup_2[]
                 ResourceConfig config = new ResourceConfig();
                 config.register(listener);
+                // end::setup_2[]
                 config.register(TestResource.class);
                 return config;
             }