In the commentarial understanding it’s neither of these.
Rather, mettacetovimutti is the arousing of mettā with all living beings as its object. The capacity to do this is held to be dependent upon the attainment of jhāna:
Now, it is by means of one of these jhānas beginning with the first that he “Dwells pervading (intent upon) one direction with his heart endued with lovingkindness, likewise the second direction, likewise the third direction, likewise the fourth direction, and so above, below, and around; everywhere and equally he dwells pervading the entire world with his heart endued with lovingkindness, abundant, exalted, measureless, free from enmity, and free from affliction” (Vibh 272; D I 250). For this versatility comes about only in one whose consciousness has reached absorption in the first jhāna and the rest.
(Visuddhimagga, IX 44)
but is not itself effected while one is in jhāna, but rather, after emergence from it.
I think neither too as according to the following suttas metta seems to depend on first jhana as REddison has said. According to Ne36 the nearest dependency to metta is faculty restraint(indriyasaṁvaro).
In MN43 limitless heart release (appamāṇā cetovimutti) is defined as:
Firstly, a mendicant meditates spreading a heart full of love to one direction, and to the second, and to the third, and to the fourth. In the same way above, below, across, everywhere, all around, they spread a heart full of love to the whole world—abundant, expansive, limitless, free of enmity and ill will.
They meditate spreading a heart full of compassion …
They meditate spreading a heart full of rejoicing …
They meditate spreading a heart full of equanimity to one direction, and to the second, and to the third, and to the fourth. In the same way above, below, across, everywhere, all around, they spread a heart full of equanimity to the whole world—abundant, expansive, limitless, free of enmity and ill will.
This is called the limitless release of the heart.
In Ne36 there are a lot of dependencies described for metta. In short form: → is a symbol for the sentence: A bhāvito bahulīkato B paripūreti (When A is cultivated(bhāvito: lit. caused to be) a lot it fills up B) 1st way → 1st mindfulness (body) → 1st jhana → 1st abiding(vihāro) → 1st right effort → letting go of conceit(mānappahānaṁ) → truth foundation(saccādhiṭṭhānaṁ) → 1st idhi based on interest(chandasamādhi) → faculty restraint(indriyasaṁvaro) → mettā.
Liberation of mind by metta, can be used as foundation on which insight takes place:
“Here, bhikkhus, some person dwells pervading one quarter with a mind imbued with loving-kindness, likewise the second quarter, the third quarter, and the fourth quarter. Thus above, below, across, and everywhere, and to all as to himself, he dwells pervading the entire world with a mind imbued with loving-kindness, vast, exalted, measureless, without enmity, without ill will. He contemplates whatever phenomena there pertain to form, feeling, perception, determinations, and consciousness as impermanent, as suffering, as a disease, as a boil, as a dart, as misery, as an affliction, as alien, as disintegrating, as empty, as non-self. With the breakup of the body, after death, he is reborn in companionship with the devas of the pure abodes. This is a rebirth not shared with worldlings.
Also it can be taken as an entry to jhanas.
By itself “liberation” here describes temporary suppression of defilements, so one can develop it and at the same time remain puthujjana. But the same can be said about jhanas.
