The poet does not rely on one figure of speech. Qabr ... ka-qabr supplies an explicit comparison; al-ʿaysh kanzan makes lifespan into a diminishing treasure; al-mawt yaʿtām gives death the behavior of a selector; the final verse likens mortality to a loose but controlled tether.
ImageRhetorical function
Grave ... like a graveShared outward end
Life as treasureDiminishing time capital
Death selectingPersonification
The slackened tetherFreedom within a limit
Several distinct figures thus explore the same governing idea: an unavoidable end.