Use more sensible defaults for OpenTsdbGaugeWriter's timeouts

Previously, the default RestTemplate that is used OpenTsdbGaugeWriter
was not used with its default configuration. Notably this meant that
it would have infinite connect and read timeouts. This is problematic
as it can cause metric writing to hang and block the scheduler for
performing any other tasks.

This commit updates OpenTsdbGaugeWriter to use a default connect
timeout of 10 seconds and a default read timeout of 30 seconds. A
constructor has been added to ease the configuration of these
timeouts. The existing option of providing your own RestTemplate
(via setRestTemplate) remains.

Closes gh-4698

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbGaugeWriter.java

@@ -31,6 +31,7 @@ import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
+import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.scheduling.annotation.Scheduled;
 import org.springframework.web.client.RestOperations;
 import org.springframework.web.client.RestTemplate;
@@ -49,9 +50,13 @@ import org.springframework.web.client.RestTemplate;
  */
 public class OpenTsdbGaugeWriter implements GaugeWriter {
 
+	private static final int DEFAULT_CONNECT_TIMEOUT = 10000;
+
+	private static final int DEFAULT_READ_TIMEOUT = 30000;
+
 	private static final Log logger = LogFactory.getLog(OpenTsdbGaugeWriter.class);
 
-	private RestOperations restTemplate = new RestTemplate();
+	private RestOperations restTemplate;
 
 	/**
 	 * URL for POSTing data. Defaults to http://localhost:4242/api/put.
@@ -74,6 +79,28 @@ public class OpenTsdbGaugeWriter implements GaugeWriter {
 
 	private OpenTsdbNamingStrategy namingStrategy = new DefaultOpenTsdbNamingStrategy();
 
+	/**
+	 * Creates a new {@code OpenTsdbGauageWriter} with the default connect (10 seconds)
+	 * and read (30 seconds) timeouts.
+	 */
+	public OpenTsdbGaugeWriter() {
+		this(DEFAULT_CONNECT_TIMEOUT, DEFAULT_READ_TIMEOUT);
+	}
+
+	/**
+	 * Creates a new {@code OpenTsdbGaugeWriter} with the given millisecond
+	 * {@code connectTimeout} and {@code readTimeout}.
+	 *
+	 * @param connectTimeout the connect timeout in milliseconds
+	 * @param readTimeout the read timeout in milliseconds
+	 */
+	public OpenTsdbGaugeWriter(int connectTimeout, int readTimeout) {
+		SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
+		requestFactory.setConnectTimeout(connectTimeout);
+		requestFactory.setReadTimeout(readTimeout);
+		this.restTemplate = new RestTemplate(requestFactory);
+	}
+
 	public RestOperations getRestTemplate() {
 		return this.restTemplate;
 	}