字段路径可以使用 (backtick). The only character that needs
to be escaped within a quoted field path is the backtick character itself,
escaped using a backslash. Special characters in field paths that
must be quoted include:*,.,
``` (backtick),[,]` 以及任何 ASCII 符号字符引用。
示例:(注意:此处的注释采用 Markdown 语法编写,因此还有一个额外的反引号层来表示代码块)\address.city`represents a field namedaddress.city, not the map key
cityin the fieldaddress.
`*`represents a field named*`,而不是任何字段。
特殊的 Field 包含所有字段的默认索引设置。此字段的资源名称为 projects/{projectId}/databases/{databaseId}/collectionGroups/__default__/fields/*。在此 Field 上定义的索引将应用于没有专属 Field 索引配置的所有字段。