Added remoteServiceName to Span interface; fixes gh-1901

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/Span.java

@@ -89,6 +89,16 @@ public interface Span extends SpanCustomizer {
 	 */
 	void abandon();
 
+	/**
+	 * Sets the remote service name for the span.
+	 * @param remoteServiceName remote service name
+	 * @return this span
+	 * @since 3.0.3
+	 */
+	default Span remoteServiceName(String remoteServiceName) {
+		return this;
+	}
+
 	/**
 	 * Type of span. Can be used to specify additional relationships between spans in
 	 * addition to a parent/child relationship.

spring-cloud-sleuth-autoconfigure/src/test/java/org/springframework/cloud/sleuth/autoconfig/NoOpSpan.java

@@ -72,4 +72,9 @@ class NoOpSpan implements Span {
 
 	}
 
+	@Override
+	public Span remoteServiceName(String remoteServiceName) {
+		return this;
+	}
+
 }

spring-cloud-sleuth-brave/src/main/java/org/springframework/cloud/sleuth/brave/bridge/BraveSpan.java

@@ -86,6 +86,12 @@ public class BraveSpan implements Span {
 		this.delegate.abandon();
 	}
 
+	@Override
+	public Span remoteServiceName(String remoteServiceName) {
+		this.delegate.remoteServiceName(remoteServiceName);
+		return this;
+	}
+
 	@Override
 	public String toString() {
 		return this.delegate != null ? this.delegate.toString() : "null";