add health check endpoint for server

Author: yunhan0

server/src/api/health-check/index.js

@@ -0,0 +1,21 @@
+'use strict'
+
+const Router = require('koa-router')
+const mongoose = require('mongoose')
+
+let router = new Router()
+
+router
+  .get('/health-check', async (ctx) => {
+    const state = mongoose.connection.readyState
+    // When state === 0, it means the connection is disconnected
+    if (!state) {
+      throw new Error('connection to mongodb is broken')
+    }
+
+    ctx.status = 204
+    ctx.body = null
+  })
+
+
+module.exports = router