code: private static Schema.Field partitionField(FieldPartitioner<?, ?> fp,
                                               Schema schema) {
      return new Schema.Field(
          fp.getName(), partitionFieldSchema(fp, schema), null, null);
    }
docs: Builds a Schema.Field for the FieldPartitioner using the Schema to
  determine types not fixed by the FieldPartitioner.
  
  @param fp a FieldPartitioner
  @param schema an entity Schema that will be partitioned
  @return a Schema.Field for the field partitioner, with the same name