thanos-store memcached errors

Problem

thanos-store in gprd is logging memcache: unexpected response line from "set": "ERROR Too many open connections\r\n" millions of times per day: https://log.gprd.gitlab.net/goto/c8ed7da5f92552b7f4132f6a80d87952

This indicates a problem, and also jams up our logging cluster.

Desired outcome

Acceptance criteria

• ...