Metronome is associated with the main transport, not the phrase. So, phrase playback should not affect the metronome at all.
At least, if I have the metronome going, it doesn't matter if I trigger a phrase from the phrase editor or via MIDI keyboard.
But I guess you're using SPACE to stop playback? This will indeed stop the main transport and thus, also the metronome.
Instead, I would suggest you use the phrase playback button (Return), which toggles between start/stop or assign the (by default unassigned) dedicated shortcut to stop phrase playback.
Also, phrase looping should apply to playback and is controlled with the phrase editor toolbar - perhaps it was simply disabled for your phrase?