log: change log format to JSON payload for better log in Stackdriver (#66)

change the log format in Python and Node.js services.

Effected services are currencyservice, emailservice, paymentservice,
and recommendationservice. Loadgenerator is left as is because of
the diffculty to change the log format and log target in locust.

ref. #47

src/emailservice/email_client.py

@@ -19,6 +19,9 @@ import grpc
 import demo_pb2
 import demo_pb2_grpc
 
+from logger import getJSONLogger
+logger = getJSONLogger('emailservice-client')
+
 # from opencensus.trace.tracer import Tracer
 # from opencensus.trace.exporters import stackdriver_exporter
 # from opencensus.trace.ext.grpc import client_interceptor
@@ -39,10 +42,10 @@ def send_confirmation_email(email, order):
       email = email,
       order = order
     ))
-    print('Request sent.')
+    logger.info('Request sent.')
   except grpc.RpcError as err:
-    print(err.details())
-    print('{}, {}'.format(err.code().name, err.code().value))
+    logger.error(err.details())
+    logger.error('{}, {}'.format(err.code().name, err.code().value))
 
 if __name__ == '__main__':
-  print('Client for email service.')
+  logger.info('Client for email service.')