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/paymentservice/charge.js

@@ -14,6 +14,15 @@
 
 const cardValidator = require('simple-card-validator');
 const uuid = require('uuid/v4');
+const pino = require('pino');
+
+const logger = pino({
+  name: 'paymentservice-charge',
+  messageKey: 'message',
+  changeLevelName: 'severity',
+  useLevelLabels: true
+});
+
 
 class CreditCardError extends Error {
   constructor (message) {
@@ -67,7 +76,7 @@ module.exports = function charge (request) {
   const { credit_card_expiration_year: year, credit_card_expiration_month: month } = creditCard;
   if ((currentYear * 12 + currentMonth) > (year * 12 + month)) { throw new ExpiredCreditCard(cardNumber.replace('-', ''), month, year); }
 
-  console.log(`Transaction processed: ${cardType} ending ${cardNumber.substr(-4)} \
+  logger.info(`Transaction processed: ${cardType} ending ${cardNumber.substr(-4)} \
     Amount: ${amount.currency_code}${amount.units}.${amount.nanos}`);
 
   return { transaction_id: uuid() };