TypeAnnotationCode is used as a part of [Type][google.spanner.v1.Type] to
disambiguate SQL types that should be used for a given Cloud Spanner value.
Disambiguation is needed because the same Cloud Spanner type can be mapped to
different SQL types depending on SQL dialect. TypeAnnotationCode doesn't
affect the way value is serialized.
PostgreSQL compatible NUMERIC type. This annotation needs to be applied to
[Type][google.spanner.v1.Type] instances having [NUMERIC][google.spanner.v1.TypeCode.NUMERIC]
type code to specify that values of this type should be treated as
PostgreSQL NUMERIC values. Currently this annotation is always needed for
[NUMERIC][google.spanner.v1.TypeCode.NUMERIC] when a client interacts with PostgreSQL-enabled
Spanner databases.
