保持使用全局 HttpClient，但确保不同请求的 HttpRequestMessage 独立

保持使用全局 HttpClient，但确保不同请求的 HttpRequestMessage 独立
这是推荐的最佳实践，因为 HttpClient 是设计为可重用的，你可以使用独立的 HttpRequestMessage 来确保每个请求有独立的请求头，而不影响其他请求。

        private static readonly HttpClient client = new HttpClient();
        
           var str = JsonConvert.SerializeObject(vo);
   var content = new StringContent(str,
       Encoding.UTF8, "application/json");

   using (var request = new HttpRequestMessage(HttpMethod.Post, site.Url))
   {
       request.Content = content;

       request.Headers.Add("appUserKey", appUserKey);
       request.Headers.Add("nonce", nonce);
       request.Headers.Add("timestamp", timestampStr);
       request.Headers.Add("sign", signature);

       ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
       LogHelper.Info($"{DateTime.Now}: PostAsync {site.Name}({site.Url})");
       var response = await client.SendAsync(request);
       if (response.IsSuccessStatusCode)
       {
           LogHelper.Info($"{DateTime.Now}:  推送数接口返回 {site.Name}({site.Url})  ({await response.Content.ReadAsStringAsync()})");
       }
       else
       {
           LogHelper.Info($"{DateTime.Now}:  推送数据出错 {site.Name}({site.Url}) , 状态码: {response.StatusCode}");
       }
   }