Motivation

As a developer and tester, I would like to capture and save QML profiler data during the execution of the application

Potential use-cases 

1. Tester downloaded and installed the latest QML-based application on his device: desktop, mobile, or car
2. Tester observed a performance issue and would like to collect QML profiler traces
3. Instead of attaching qmlprofiler to the app process, the tester performs some "secret" action to initiate capturing of qml profiler data
4. The application code initiates the capturing of qml profiler data during the next XX seconds
5. After the timeout, the app saves the data on a disk

Proposal

Extend QQmlEngine API

class QQmlEngine {
public:
    ...
#if QT_CONFIG(qml_debug)
    void captureProfiledData(int msec, const QString& filePath);
#endif
} 

Example of usage

// somewhere in my code

connect(&app, MyApp::profileDataRequested, this, [] (){
    // collect qmlprofile data during next 60 seconds 
    // and save it on a disk 
    qmlEngine()->captureProfiledData(60000, QStringLiteral("/tmp/traces.qzt"));
})