Log stacktrace on additional metadata merge fail

See gh-2288

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

@@ -22,6 +22,8 @@ import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.io.PrintWriter;
+import java.io.StringWriter;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -367,10 +369,17 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 		}
 		catch (Exception ex) {
 			logWarning("Unable to merge additional-spring-configuration-metadata.json");
+			logWarning(getStackTrace(ex));
 			return metadata;
 		}
 	}
 
+	private String getStackTrace(Exception ex) {
+		StringWriter writer = new StringWriter();
+		ex.printStackTrace(new PrintWriter(writer, true));
+		return writer.toString();
+	}
+
 	private InputStream getAdditionalMetadata() throws IOException {
 		// Most build systems will have copied the file to the class output location
 		FileObject fileObject = this.processingEnv.getFiler().createResource(