I did not find the standard function to get the frame rate in C++ so I get them this way:
Tick execute in second so many times how many frames per second is displayed, so you can count how many times was called tick per second using a timer.

void AGameCharacter::BeginPlay()
{
Super::BeginPlay();
FTimerHandle FPSTimerHandle;
GetWorld()->GetTimerManager().SetTimer(FPSTimerHandle, this, &AGameCharacter::ShowFrameRate, 1.f, true);
}
int FPS = 0;
void AGameCharacter::Tick( float DeltaTime )
{
Super::Tick(DeltaTime);
FPS++;
}
void AGameCharacter::ShowFrameRate()
{
GEngine->AddOnScreenDebugMessage(0, 1.f, FColor::Yellow, "FPS: " + FString::FromInt(FPS) + " ms: " + FString::SanitizeFloat(FApp::GetDeltaTime() * 1000));
FPS = 0;
}