double previousTime = glfwGetTime();
int frameCount = 0;

while (isGameRunning)
{
    // Measure speed
    double currentTime = glfwGetTime();
    frameCount++;
    // If a second has passed.
    if ( currentTime - previousTime >= 1.0 )
    {
        // Display the frame count here any way you want.
        displayFPS(frameCount);

        frameCount = 0;
        previousTime = currentTime;
    }

    // Rest of your game loop...
 }