@Override
    public String toString() {
      return "TypeAdapters.timeInstantAdapter()";
    }
Use this adapter (not registered by factory, default Gson adapter is used otherwise) to
delagate marshaling of {@link BigDecimal} to {@link Decimal128} type in mongo.
@return {@code BigDecimal <==> Decimal128} adapter
mongo/src/org/immutables/mongo/types/TypeAdapters.java