Update default value for Datadog's default uri

See gh-20833

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/datadog/DatadogProperties.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * 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.
@@ -56,7 +56,7 @@ public class DatadogProperties extends StepRegistryProperties {
 	 * URI to ship metrics to. If you need to publish metrics to an internal proxy
 	 * en-route to Datadog, you can define the location of the proxy with this.
 	 */
-	private String uri = "https://app.datadoghq.com";
+	private String uri = "https://api.datadoghq.com";
 
 	public String getApiKey() {
 		return this.apiKey;