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## Why
Current pool metrics show checkout outcomes and pool state after the
fact. They do not show when a checkout began or which writer-connection
setup step failed. During startup and pool growth, operators need to
distinguish pool saturation from a slow or unsafe connection setup.
## What
- Count every instrumented database checkout when it starts, with a
fixed operation label.
- Measure writer-pool creation and every physical writer connection
across `physical_connect`, `created_at_floor`, `session_timeouts`,
`isolation`, and `ready`.
- Publish fixed-cardinality start, terminal-outcome, duration, and
waiter metrics without database URLs, SQL, raw errors, connection
ordinals, or per-connection lifecycle logs.
- Document how to identify the active setup bottleneck during a rollout
or incident.
## How
The existing typed checkout wrapper records starts and current waiters.
The huddle-history path uses that wrapper instead of a raw pool
checkout.
The production SQLx `after_connect` hook records start, terminal
outcome, and duration for each sequential safety step. A drop guard
records cancellation once if setup exits before a terminal result. Fixed
enums keep every label bounded.
For a phase, `started_total - sum(attempts_total)` is the number of
in-progress attempts on that pod. Because setup is sequential, a later
phase starting also proves the earlier phases succeeded.
The database path emits metrics only. This PR intentionally does not add
per-connection lifecycle receipts or direct `stderr` writes.
## Risk
Medium. This changes the production writer-pool `after_connect` hook and
adds one counter update to instrumented checkout paths. It does not
change the database safety statements or their failure behavior. The
telemetry has fixed labels and no per-connection identifiers.
## Testing
At exact head `ffb5fcb114a9c986ffd0cb2cfc9d413a32d5b1aa`, a release
relay was started against isolated local PostgreSQL, Redis, and MinIO
services. The main health endpoint returned `ok`, readiness returned
`{"status":"ready"}`, the connection-step metrics were exported, and no
database lifecycle receipts were emitted. A release CLI then created a
channel, sent a message, and read the same event back successfully.
The earlier staging deployment used pre-rebase head
`f58e9480a4f068db0c591f604fd6800fdd4bfc45`. The deployed
multi-architecture image came from [GitHub Actions run
33780888255](https://github.com/block/buzz/actions/runs/33780888255),
manifest
`sha256:b9351fa644e08376cbe1999f9bee311d33d1799a68eadef7929c4f862a832fec`.
The [staging
deployment](https://github.com/squareup/builderbot-platform-core-infrastructure/pull/317)
brought both pods in ReplicaSet `buzz-6c8758bd7d` to Ready with zero
restarts, and the connection metrics produced data in the [rollout
dashboard](https://app.datadoghq.com/dashboard/tm7-qxr-wt2/buzz-startup--rollout-safety).
That staging image predates the cleanup that removed per-connection
lifecycle logs; the metric schema and database safety statements are
unchanged.
## Verification
- `cargo test -p buzz-db -- --test-threads=1`: 128 tests passed across
the package and integration target; 255 opt-in tests remained ignored.
- Four opt-in production-path PostgreSQL regressions passed: initial
minimum connections, post-startup pool growth, isolation failure, and
session-timeout setup failure.
- All 35 relay media tests passed against the isolated PostgreSQL
database.
- Rust formatting, workspace clippy, desktop/Tauri clippy, web checks,
mobile analysis, security checks, and file-size policy checks passed.
- The repository-wide unit stage also exposed three unrelated existing
`buzz-acp` failures: one timing-sensitive keepalive test and two
environment-default tests.
## Bigger picture
This is the database-metrics part of the startup and rollout
observability work. The early-startup lifecycle foundation merged in
#7258, so the rebase removed that duplicate commit from this PR.
Originating discussion:
buzz://message?channel=6ac85131-70cd-4bda-a031-38d34114934e&id=fa2bed181c092697210a60bb6eedc55a665d5c1c6cabc1413a04686647011f71
Generated with Codex
---------
Signed-off-by: Ravneet Arora <rarora@squareup.com>