Case C-377/21: Judgment of the Court (Seventh Chamber) of 7 July 2022 (request for a preliminary ruling from the Cour du travail de Mons — Belgium) — Ville de Mons, Zone de secours Hainaut-Centre v RM (Reference for a preliminary ruling — Social policy — Directive 97/81/EC — Framework Agreement on part-time work — Clause 4 — Principle of non-discrimination — Principle of pro rata temporis — Taking into account, for the purpose of calculating the remuneration of a full-time professional firefighter, the length of service for remuneration purposes which he has acquired as a volunteer firefighter, in accordance with the principle of pro rata temporis)