The process whose specific outcome is the progression of the metanephric long descending thin limb over time, from its formation to the mature structure. The metanephric long descending thin limb is the descending thin limb of a long nephron in the metanephros that has a squamous epithelial morphology. The long descending limb starts in the inner stripe of the outer medulla and extends into the inner medulla. [ GOC:mtg_kidney_jan10 ]