Helper to get trace_id from web application request header.

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

def get_trace_id():
    """Helper to get trace_id from web application request header.

    :rtype: str
    :returns: TraceID in HTTP request headers.
    """
    checkers = (
        get_trace_id_from_django,
        get_trace_id_from_flask,
        get_trace_id_from_webapp2,
    )

    for checker in checkers:
        trace_id = checker()
        if trace_id is not None:
            return trace_id

    return None