Some valid field names are considered invalid as a FieldPath (for example, fields like "foo.$bar" where a sub-component is prefixed with "$"). Currently, if serializeFieldPath errors due to supposedly "invalid" field names, we instead serialize that field name with a placeholder string.

We should consider options to redact field names that are considered invalid to the FieldPath validation and remove any use of a placeholder string. The most obvious albeit non-optimal solution would be to manually split the string into its sub-components, then (as is done in serializeFieldPath) run the redaction policy over each component and reconstruct the redacted field.