feat: intra-batch anon stats for HNSW

[dkales]

First stab at adding intra-batch distances to anon stats. As a slight complication, like the matching itself, anon stats for intra-batch matches are only added if the matching element would actually be inserted into the DB.

Just tested locally for now, since the anon stats E2E tests assume pretty heavily right now that there is a batch size of 1.