Add support for reactive Elasticsearch healthcheck

Prior to this commit, configuring a reactive Elasticsearch client would
auto-configure an Actuator Health check using a synchronous client, with
the default configuration properties (so tarting localhost:9200).

This would lead to false reports of unhealthy Elasticsearch clusters
when using reactive clients.

This commit reproduces the logic for MongoDB repositories: if a reactive
variant is available, it is selected for the health check
infrastructure.

See gh-21042

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/elasticsearch/ElasticsearchReactiveHealthIndicator.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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 org.springframework.boot.actuate.elasticsearch;
+
+import java.util.stream.Collectors;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.boot.actuate.health.AbstractReactiveHealthIndicator;
+import org.springframework.boot.actuate.health.Health;
+import org.springframework.boot.actuate.health.HealthIndicator;
+import org.springframework.data.elasticsearch.client.reactive.ReactiveElasticsearchClient;
+
+/**
+ * {@link HealthIndicator} for an Elasticsearch cluster using a
+ * {@link ReactiveElasticsearchClient}.
+ *
+ * @author Aleksander Lech
+ * @since 2.3
+ */
+public class ElasticsearchReactiveHealthIndicator extends AbstractReactiveHealthIndicator {
+
+	private final ReactiveElasticsearchClient client;
+
+	public ElasticsearchReactiveHealthIndicator(ReactiveElasticsearchClient client) {
+		super("Elasticsearch health check failed");
+		this.client = client;
+	}
+
+	@Override
+	protected Mono<Health> doHealthCheck(Health.Builder builder) {
+		return this.client.status().map((status) -> {
+			if (status.isOk()) {
+				builder.up();
+			}
+			else {
+				builder.down();
+			}
+
+			builder.withDetails(status.hosts().stream().collect(Collectors
+					.toMap((host) -> host.getEndpoint().getHostString(), (host) -> host.getState().toString())));
+
+			return builder.build();
+		});
+	}
+
+}