## Problem
Previously, we would run db migrations before doing the step-down
sequence. This meant that the current leader would have to deal with
the schema changes and that's generally not safe.
## Summary of changes
Push the step-down procedure earlier in start-up and
do db migrations right after it (but before we load-up the in-memory
state from the db).
Epic: https://github.com/neondatabase/cloud/issues/14701
## Problem
Storage controller restarts cause temporary unavailability from the
control plane POV. See RFC for more details.
## Summary of changes
* A couple of small refactors of the storage controller start-up
sequence to make extending it easier.
* A leader table is added to track the storage controller instance
that's currently the leader (if any)
* A peer client is added such that storage controllers can send
`step_down` requests to each other (implemented in
https://github.com/neondatabase/neon/pull/8512).
* Implement the leader cut-over as described in the RFC
* Add `start-as-candidate` flag to the storage controller to gate the
rolling restart behaviour. When the flag is `false` (the default), the
only change from the current start-up sequence is persisting the leader
entry to the database.
## Problem
We are missing the step-down primitive required to implement rolling
restarts of the storage controller.
## Summary of changes
Add `/control/v1/step_down` endpoint which puts the storage controller
into a state where it rejects
all API requests apart from `/control/v1/step_down`, `/status` and
`/metrics`. When receiving the request,
storage controller cancels all pending reconciles and waits for them to
exit gracefully. The response contains
a snapshot of the in-memory observed state.
Related:
* https://github.com/neondatabase/cloud/issues/14701
* https://github.com/neondatabase/neon/issues/7797
* https://github.com/neondatabase/neon/pull/8310
## Problem
Some awkwardness in the measured API.
Missing process metrics.
## Summary of changes
Update measured to use the new convenience setup features.
Added measured-process lib.
Added measured support for libmetrics
The binary etc were renamed some time ago, but the path in the source
tree remained "attachment_service" to avoid disruption to ongoing PRs.
There aren't any big PRs out right now, so it's a good time to cut over.
- Rename `attachment_service` to `storage_controller`
- Move it to the top level for symmetry with `storage_broker` & to avoid
mixing the non-prod neon_local stuff (`control_plane/`) with the storage
controller which is a production component.
