[AzHicham]

Some SQLx APIs cannot take an instrumented Pool today:

• sqlx::migrate::Migrator::run is generic over sqlx::Acquire, which &Pool does not implement;
• sqlx::postgres::PgListener::connect_with accepts only a sqlx::Pool, and the wrapped pool is private.

An application that wraps its pool therefore has to keep a second handle to the raw pool next to it, just for migrations and LISTEN/NOTIFY.

Changes

• impl sqlx::Acquire for &Pool<DB>. Both methods acquire through Pool::acquire, so wait_time, pending_requests and timeouts are recorded as on every other path.
  • acquire returns a new AcquiredConnection<DB>, which derefs to the backend connection (what Acquire::Connection requires) and records use_time when dropped.
  • begin returns a plain sqlx::Transaction. It cannot report when it releases its connection, so that lease records no use_time rather than a wrong one.
  • Queries run on that connection or transaction are not instrumented, since they no longer go through the wrapper.
• Pool::inner() -> &sqlx::Pool<DB>, for APIs that accept nothing else. Nothing done through it is instrumented.

A separate AcquiredConnection type, rather than Deref on PoolConnection, keeps &mut *conn from silently bypassing instrumentation on the connection type users already have.

Docs, README and CHANGELOG cover both, including what is not instrumented.

Tests

• pool_metrics.rs: Acquire::acquire (wait and use time, no spans), Acquire::begin (wait time only), Migrator::run(&pool), and inner() (no spans, no wait time).
• postgres.rs: PgListener::connect_with(pool.inner()) receives a NOTIFY.
• cargo test --all-features, the async-std pool_metrics run, clippy -D warnings and fmt pass locally.

sqlx/migrate is added to the dev-dependencies for the Migrator test.

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