'Create tracker.
This class is used to track a gzipped filehandle for each type of output
(e.g. ExportedFile) during ProcessResponses. Then during Flush the data
from the temp file is sent to bigquery. Flush is guaranteed to be called on
the same worker so holding local file references is OK.
Args:
output_type: string, e.g. "ExportedFile"
gzip_filehandle: open handle to a gzip.GzipFile opened on
gzip_filehandle_parent. JSON data will be written here.
gzip_filehandle_parent: open parent filehandle for gzip_filehandle.
Reading from this handle gives you the gzip content.
schema: Bigquery schema, array of dicts'

def __init__(self, output_type=None, gzip_filehandle=None, gzip_filehandle_parent=None, schema=None):

    self.output_type = output_type
    self.gzip_filehandle = gzip_filehandle
    self.schema = schema
    self.gzip_filehandle_parent = gzip_filehandle_parent
