bff4a11b3f
Re-introduce: https://github.com/element-hq/synapse/pull/18751 that was reverted in https://github.com/element-hq/synapse/pull/18789 (explains why the PR was reverted in the first place). - Adds a `cleanup` pattern that cleans up metrics from each homeserver in the tests. Previously, the list of hooks built up until our CI machines couldn't operate properly, see https://github.com/element-hq/synapse/pull/18789 - Fix long-standing issue with `synapse_background_update_status` metrics only tracking the last database listed in the config (see https://github.com/element-hq/synapse/pull/18791#discussion_r2261706749)