fix(mongodb): add default values

Author: dszakallas

lib/instrumentations/mongodb.js

@@ -46,7 +46,7 @@ module.exports = function (mongodb, agent) {
       return utils.wrapQuery.call(this, original, args, agent, {
         protocol: consts.PROTOCOLS.MONGODB,
         url: _this.collectionName || 'unknown',
-        host: host,
+        host: host || 'unknown',
         method: name,
         disableCallback: name === 'aggregate'
       })