Menger curvature and rectifiability in metric spaces

We show that for any metric space $X$ the condition $${\int }_X{\int }_X{\int }_{X}c(z_1,z_2,z_3{)}^{2}d\text{Hm}{z}_{1}d\text{Hm}{z}_{2}d\text{Hm}{z}_3<\mathrm{\infty },$$ where $c(z_1,z_2,z_3)$ is the Menger curvature of the triple $(z_1,z_2,z_3)$, guarantees that $X$ is rectifiable.