rename trace field for googleCloudLogging (#183)

Author: J-Rocke

helper.go

@@ -70,9 +70,11 @@ func SetWithConfig(level string, config *LogConfig) error {
 	} else if config.googleCloudLogging {
 		logger.Formatter = &logrus.JSONFormatter{
 			FieldMap: logrus.FieldMap{
+				// https://cloud.google.com/logging/docs/agent/logging/configuration#special-fields
 				logrus.FieldKeyTime:  "timestamp",
 				logrus.FieldKeyLevel: "severity",
 				logrus.FieldKeyMsg:   "message",
+				"trace":              "traceID", // Trace is special in GCE so we have to use another name.
 			},
 			TimestampFormat: time.RFC3339Nano,
 		}