Add MDC support in Logback appender

Author: dietervdw-spotify

google-cloud-clients/google-cloud-contrib/google-cloud-logging-logback/src/main/java/com/google/cloud/logging/logback/LoggingAppender.java

@@ -36,6 +36,7 @@
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 /**
@@ -232,6 +233,10 @@ private LogEntry logEntryFor(ILoggingEvent e) {
         .addLabel(LEVEL_NAME_KEY, level.toString())
         .addLabel(LEVEL_VALUE_KEY, String.valueOf(level.toInt()));
 
+    for (Map.Entry<String, String> entry : e.getMDCPropertyMap().entrySet()) {
+      builder.addLabel(entry.getKey(), entry.getValue());
+    }
+
     if (loggingEnhancers != null) {
       for (LoggingEnhancer enhancer : loggingEnhancers) {
         enhancer.enhanceLogEntry(builder);