Add json out flag to qps driver

test/cpp/qps/qps_json_driver.cc

@@ -32,6 +32,7 @@
  */
 
 #include <iostream>
+#include <fstream>
 #include <memory>
 #include <set>
 
@@ -72,6 +73,9 @@ DEFINE_string(qps_server_target_override, "",
               "Override QPS server target to configure in client configs."
               "Only applicable if there is a single benchmark server.");
 
+DEFINE_string(json_file_out, "",
+              "File to write the JSON output to.");
+
 namespace grpc {
 namespace testing {
 
@@ -103,6 +107,13 @@ static std::unique_ptr<ScenarioResult> RunAndReport(const Scenario& scenario,
     *success = result->server_success(i);
   }
 
+  if (FLAGS_json_file_out != "") {
+    std::ofstream json_outfile;
+    json_outfile.open(FLAGS_json_file_out);
+    json_outfile << "{\"qps\": " << result->summary().qps() << "}\n";
+    json_outfile.close();
+  }
+
   return result;
 }