Mettā Loving Kindness and 3rd Satipaṭṭhāna - Ajahn Dhammasiha
Loving Kindness (Mettā), the intention of unconditional, non-judgemental good will to all beings without exception, is generally considered a meditation object mostly to develop samādhi (concentration), and not really suitable for developing insight (vipassana).
However, in his reflections Ajahn Dhammasiha provides some pointers how we can practise the Third Foundation of Mindfulness (3rd Satipaṭṭhāna), Contemplation of Mind (Cittānupassanā), while developing Loving Kindness:
We mindfully observe if the emotion of Mettā is present throughout our daily life ("Sadosaṃ/vītadosaṃ cittan'ti pajānāti"). We carefully observe how our Mettā changes depending on the conditions we encounter. For instance, if someone treats us in an unfriendly way, our Mettā becomes weaker, or may even turn into irritation. Thus we can observe "phenomena of arising and passing away in regard to mindstates" ("Samudaya-vaya-dhammānupassī vā cittasmiṃ viharati") while trying to sustain the emotion of Loving Kindness throughout the day.