feat(topology/metric_space/hausdorff_distance): The diameter of a thickening (#17021)
feat(topology/metric_space/hausdorff_distance): The diameter of a thickening (#17021)

The diameter of the `ε`-thickening of `s` is at most the diameter of `s` + `2ε`





leanprover-community/mathlib,leanprover-community/mathlib