Allow tls_auth_handler to support client side verification only

The tls_auth_handler supports setting up a mTLS connection with the Prometheus push gateway, but I want only to verify the certificate of the server. In other words, I wish to skip setting certfile and keyfile in the tls_auth_handler method when the protocol is not ssl.PROTOCOL_TLS_SERVER.

The authentication on the server side I will handle differently.

I'd be willing to contribute. Should I add it as a conditional to the current tls_auth_handler method (think this will ), or should I create a new tls_handler method?

Add to current tls_auth_handler:

def tls_auth_handler(
        url: str,
        method: str,
        timeout: Optional[float],
        headers: List[Tuple[str, str]],
        data: bytes,
        certfile: str,
        keyfile: str,
        cafile: Optional[str] = None,
        protocol: int = ssl.PROTOCOL_TLS_CLIENT,
        insecure_skip_verify: bool = False,
) -> Callable[[], None]:
    """Handler that implements an HTTPS connection with TLS Auth.

    The default protocol (ssl.PROTOCOL_TLS_CLIENT) will also enable
    ssl.CERT_REQUIRED and SSLContext.check_hostname by default. This can be
    disabled by setting insecure_skip_verify to True.

    Both this handler and the TLS feature on pushgateay are experimental."""
    context = ssl.SSLContext(protocol=protocol)
    if cafile is not None:
        context.load_verify_locations(cafile)
    else:
        context.load_default_certs()

    if insecure_skip_verify:
        context.check_hostname = False
        context.verify_mode = ssl.CERT_NONE
    
    if protocol == ssl.PROTOCOL_TLS_SERVER:
        context.load_cert_chain(certfile=certfile, keyfile=keyfile)
    
    handler = HTTPSHandler(context=context)
    return _make_handler(url, method, timeout, headers, data, handler)

tls_auth_handler エントリポイントから開始し、その SSLContext がどのように構成されているかを、HTTPSHandler の構築を含めて確認してください。クライアント専用の検証がクライアント証明書ファイルを必要とせずに機能するようにしつつ、サーバープロトコルで使用するための証明書チェーンの読み込みは維持してください。handler が引き続き、指定されている検証モードと非セキュアモードをサポートしていることを確認してください。