Temporary fix for datadog agent tests

Author: cgilmour

test/mocks/http_clients.h

@@ -39,9 +39,12 @@ struct MockHTTPClient : public HTTPClient {
   ResponseHandler on_response_;
   ErrorHandler on_error_;
 
-  Expected<void> post(const URL&, HeadersSetter set_headers,
+  Expected<void> post(const URL& url, HeadersSetter set_headers,
                       std::string /*body*/, ResponseHandler on_response,
                       ErrorHandler on_error) override {
+    if (url.path != "/v0.4/traces") {
+      return {};
+    }
     std::lock_guard<std::mutex> lock{mutex_};
     if (!post_error) {
       on_response_ = on_response;