chore: fix auto-population monitored resource (#657)

to allow unit testing to customize the logic
auto-population should be skipped if monotored resource is set.

Author: minherz

src/main/java/com/google/cloud/logging/logback/LoggingAppender.java

@@ -233,10 +233,8 @@ public Synchronicity getWriteSynchronicity() {
   }
 
   private void setupMonitoredResource() {
-    if (autoPopulateMetadata) {
+    if (monitoredResource == null && autoPopulateMetadata) {
       monitoredResource = MonitoredResourceUtil.getResource(getProjectId(), resourceType);
-    } else {
-      monitoredResource = null;
     }
   }