When you see the error Failed to get local.oplog_rs collection stats. in your mongodb exporter logs, make sure the DB user account you use has the right permissions.

db.getSiblingDB("admin").createUser({
    user: "mongodb_exporter",
    pwd: "password",
    roles: [
        { role: "clusterMonitor", db: "admin" },
        { role: "read", db: "local" }
    ]
})

And then configure the exporter to use the mongodb_exporter user account.