A set of field paths on a document. Used to restrict a get or update
operation on a document to a subset of its fields. This is different
from standard field masks, as this is always scoped to a
Document][google.firestore.v1.Document], and takes in account the
dynamic nature of Value][google.firestore.v1.Value].
The list of field paths in the mask. See
`Document.fields][google.firestore.v1.Document.fields]` for a
field path syntax reference.