Kubernetes endpoints throw `ServiceUnavailable`
Unfortunately those URLs are incomplete. Influx's ports are named, so you need to say which port you want.
https://MASTER_IP/api/v1/proxy/namespaces/kube-system/services/monitoring-influxdb:http or https://MASTER_IP/api/v1/proxy/namespaces/kube-system/services/monitoring-influxdb:api
There's a bug open to give better errors for this.