Polish contribution

`InfoProvider` is now `InfoContributor` and contributes to the `Info`
instance via a builder. The `Info` instance is immutable. Each
contributor can be disabled via the `management.info.<name>.enabled`
key or all can be disabled using `management.info.defaults.enabled` (this
is similar to what the health endpoint does).

By default, all keys from the environment starting with `info.` are
exposed. If a `git.properties` file is present in the classpath, the
content of `GitInfo` is exposed using the `git` key.

A `SimpleInfoContributor` and `AbstractEnvironmentInfoContributor` are
available for convenience. `InfoContributor` instances can be ordered
the usual way, with a default order provided by
`InfoProviderAutoConfiguration#DEFAULT_ORDER`.

Closes gh-3492

spring-boot-samples/spring-boot-sample-actuator/src/main/java/sample/actuator/ExampleInfoContributor.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2012-2016 the original author or authors.
+ *
+ * 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 sample.actuator;
+
+import java.util.Collections;
+
+import org.springframework.boot.actuate.info.Info;
+import org.springframework.boot.actuate.info.InfoContributor;
+import org.springframework.stereotype.Component;
+
+@Component
+public class ExampleInfoContributor implements InfoContributor {
+
+	@Override
+	public void contribute(Info.Builder builder) {
+		builder.withDetail("example",
+				Collections.singletonMap("someKey", "someValue"));
+	}
+
+}

spring-boot-samples/spring-boot-sample-actuator/src/main/java/sample/actuator/ExampleInfoProvider.java

@@ -1,21 +0,0 @@
-package sample.actuator;
-
-import org.springframework.boot.actuate.info.Info;
-import org.springframework.boot.actuate.info.InfoProvider;
-import org.springframework.stereotype.Component;
-
-@Component
-public class ExampleInfoProvider implements InfoProvider {
-
-	@Override
-	public Info provide() {
-		Info result = new Info();
-		result.put("somekey", "somevalue");
-		return result;
-	}
-
-	@Override
-	public String name() {
-		return "example";
-	}
-}