Javadoc
Adds a field path to the tree. In a FieldMask, every field path matches the
specified field as well as all its sub-fields. For example, a field path
"foo.bar" matches field "foo.bar" and also "foo.bar.baz", etc. When adding
a field path to the tree, redundant sub-paths will be removed. That is,
after adding "foo.bar" to the tree, "foo.bar.baz" will be removed if it
exists, which will turn the tree node for "foo.bar" to a leaf node.
Likewise, if the field path to add is a sub-path of an existing leaf node,
nothing will be changed in the tree.