fix(tracing): align backend_pid field name on postgres.close

Review finding #5: postgres.establish records the backend PID as
`db.postgresql.backend_pid` but postgres.close recorded the same value as
plain `backend_pid`, so consumers filtering on the field name would miss
one. Use `db.postgresql.backend_pid` in both spans.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: grahamc

sqlx-postgres/src/connection/mod.rs

@@ -163,7 +163,7 @@ impl Connection for PgConnection {
         target = "sqlx::connect",
         name = "postgres.close",
         skip_all,
-        fields(db.system = "postgresql", backend_pid = self.inner.process_id),
+        fields(db.system = "postgresql", db.postgresql.backend_pid = self.inner.process_id),
         level = "debug",
     )]
     async fn close(mut self) -> Result<(), Error> {