Add Javadoc since for outcome key name enum values (#3748)

See gh-3729

Author: izeye

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/httpcomponents/ApacheHttpClientObservationDocumentation.java

@@ -52,6 +52,11 @@ public String asString() {
                 return "status";
             }
         },
+
+        /**
+         * Key name for outcome.
+         * @since 1.11.0
+         */
         OUTCOME {
             @Override
             public String asString() {

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/httpcomponents/hc5/ApacheHttpClientObservationDocumentation.java

@@ -48,6 +48,11 @@ public String asString() {
                 return "status";
             }
         },
+
+        /**
+         * Key name for outcome.
+         * @since 1.11.0
+         */
         OUTCOME {
             @Override
             public String asString() {

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/okhttp3/OkHttpObservationDocumentation.java

@@ -97,6 +97,10 @@ public String asString() {
             }
         },
 
+        /**
+         * Key name for outcome.
+         * @since 1.11.0
+         */
         OUTCOME {
             @Override
             public String asString() {

micrometer-core/src/main/java11/io/micrometer/core/instrument/binder/jdk/HttpClientObservationDocumentation.java

@@ -60,6 +60,10 @@ public String asString() {
             }
         },
 
+        /**
+         * Key name for outcome.
+         * @since 1.11.0
+         */
         OUTCOME {
             @Override
             public String asString() {