> grep conn95 mongod.log | head -2
2019-01-15T21:41:19.026+1100 I NETWORK  [conn95] received client metadata from 192.168.1.123:59354 conn95: { driver: { name: "PyMongo", version: "3.6.0" }, os: { type: "Darwin", name: "Darwin", architecture: "x86_64", version: "10.14.2" }, platform: "CPython 2.7.14.final.0", application: { name: "monitoring-agent v1.2.3-dev" } }
2019-01-15T21:41:19.027+1100 I ACCESS   [conn95] Successfully authenticated as principal monitoring-agent on admin

use admin;
db.grantRolesToUser("monitoring-agent", [{ role: "clusterMonitor", db: "admin" }]);