Get trace_id from flask request headers.

    :rtype: str
    :returns: TraceID in HTTP request headers.

def get_trace_id_from_flask():
    """Get trace_id from flask request headers.

    :rtype: str
    :returns: TraceID in HTTP request headers.
    """
    if flask is None or not flask.request:
        return None

    header = flask.request.headers.get(_FLASK_TRACE_HEADER)

    if header is None:
        return None

    trace_id = header.split("/", 1)[0]

    return trace_id