Add telemetry field for mean frametime

Previously, telemetry results couldn't give a good estimate for
performance over time, because it didn't include any fields related to
performance. With this, devs should be able to query metabase for mean
frametime to check for performance regressions after a change is made.

src/core/telemetry_session.cpp

@@ -99,6 +99,8 @@ TelemetrySession::~TelemetrySession() {
                                 std::chrono::system_clock::now().time_since_epoch())
                                 .count()};
     AddField(Telemetry::FieldType::Session, "Shutdown_Time", shutdown_time);
+    AddField(Telemetry::FieldType::Session, "Mean_Frametime_MS",
+             Core::System::GetInstance().perf_stats->GetMeanFrametime());
 
 #ifdef ENABLE_WEB_SERVICE
     auto backend = std::make_unique<WebService::TelemetryJson>(Settings::values.web_api_url,