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}");}}
