Don't rethrow ClosedSenderException; fixes gh-2199

Author: marcingrzejszczak

spring-cloud-sleuth-autoconfigure/src/main/java/org/springframework/cloud/sleuth/autoconfig/zipkin2/ZipkinAutoConfiguration.java

@@ -28,6 +28,7 @@
 import zipkin2.CheckResult;
 import zipkin2.Span;
 import zipkin2.reporter.AsyncReporter;
+import zipkin2.reporter.ClosedSenderException;
 import zipkin2.reporter.InMemoryReporterMetrics;
 import zipkin2.reporter.Reporter;
 import zipkin2.reporter.ReporterMetrics;
@@ -143,6 +144,9 @@ public void run() {
 					log.debug("Flushing done - closing the reporter");
 					asyncReporter.close();
 				}
+				catch (ClosedSenderException ex) {
+					log.debug("Sender already closed", ex);
+				}
 				catch (Exception e) {
 					throw new IllegalStateException(e);
 				}