* Adds a middleware that sets 2 dynamic vars

- These vars will be used to insert context for embedding analytics

* migration adding embedding_client and embedding_version to qe and vl tables

* clarify function api

* headers come in -> dynamic vars -> written to db

When the vars are set we insert their values into query execution and
query_log under embedding_client and embedding_version.

* update update migrations

* add predconditions to migrations

* fix tablenames + dataset_test

* Code Review responses

- remove commented out requires
- clarify test for merge-count-maps