创建了一个具有 clusterMonitor 和 dbOwner 角色的用户，但 db.getRoles() 显示了 enableSharding 角色

我创建了一个在其自己的数据库中具有完全权限的用户 ( dbOwner) 和对管理命令的只读访问权限 ( clusterMonitor)

use customerdb           
(mongod-3.4.7) customerdb> db.createUser( { user: "customer",
...                  pwd: "customerpw",
...                  roles: [ { role: "clusterMonitor", db: "admin" },
...                           { role: "dbOwner", db: "customerdb" }] },
...                { w: "majority" , wtimeout: 5000 } )        
Successfully added user: {
  "user": "customer",
  "roles": [
    {
      "role": "clusterMonitor",
      "db": "admin"
    },
    {
      "role": "dbOwner",
      "db": "customerdb"
    }
  ]
}

启用身份验证并使用新用户登录。这是 Homebrew 安装的最新版本的 MongoDB 单实例。

$ mongo -u customer -p customerpw localhost --authenticationDatabase=customerdb

为什么getRoles()显示我enableSharding的角色？我没有在文档中找到解释

> db.getRoles(
...     {
...       rolesInfo: 1,
...       showPrivileges:false,
...       showBuiltinRoles: true
...     }
... )      
[
  {
    "role": "dbAdmin",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ]
  },
  {
    "role": "dbOwner",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ]
  },
  {
    "role": "enableSharding",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ]
  },
  {
    "role": "read",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ]
  },
  {
    "role": "readWrite",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ]
  },
  {
    "role": "userAdmin",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ]
  }
]

enableSharding 角色的权限

  {
    "role": "enableSharding",
    "db": "customerdb",
    "isBuiltin": true,
    "roles": [ ],
    "inheritedRoles": [ ],
    "privileges": [
      {
        "resource": {
          "db": "",
          "collection": ""
        },
        "actions": [
          "enableSharding"
        ]
      }
    ],
    "inheritedPrivileges": [
      {
        "resource": {
          "db": "",
          "collection": ""
        },
        "actions": [
          "enableSharding"
        ]
      }
    ]
  }

我在带有以下版本的 mongos 上的分片集群中对此进行了测试：

MongoDB Enterprise mongos> db.version()
3.2.11

以及在具有单个 mongod 和版本 3.4.7 的 MacBook 上

我想我在创建用户和授予角色方面做错了什么？