Hey!
So far I see you doing it right making it te::Plugin.
Look at Midi RecordingDemo in examples. It shows how recording works.
I think what is wrong is position of you Plugin on the te::Track. You Plugin produces Midi and they are played by plugin positioned after your NoteRepeter Plugin.
So Midi is played but not recorded. I would advice you to checkout how you can record Midi produced by Arperggiator plugin in Waveform.
Also checkout this thread. I ran into same issues in the past. Configure Track A as MidiTrack input for Track B