Currently, IndexAccessMethod tracks multikey paths as an std::vector<std::set<std::size_t>> with size equal to the number of elements in the index key pattern. Each element in the vector is an ordered set of path-component indices that represent which path-prefixes of the corresponding indexed field cause the index to be multikey.

For allPaths indexes, we do not know how many elements are in the key pattern, and multikey paths are instead represented in the form {{{ 1: "path.to.array" }}}. We must rework the MultikeyPaths structure to enable it to accept metadata in this format.