Check host load before creating the pgbench instance

Why: A timed window that starts under load average above 16 is not a usable baseline, and creating the instance first only to pause wastes a startup.
Evidence: The rerun at load 24.03 exited 3 before any timed window and removed the instance.
Impact: The pause now happens before initdb. No change to the benchmark itself.
Refs: docs/release-multi-instance.md B6
Co-authored-by: Cursor <cursoragent@cursor.com>

tests/load/collaboration/run_pgbench.sh

@@ -130,7 +130,8 @@ fi
 
 command -v pgbench >/dev/null 2>&1 || die "pgbench is not installed"
 
-# 计时窗口才受负载门槛约束。生命周期自检（--hold-after-start）不测量吞吐。
+# 计时窗口才受负载门槛约束，并且在创建实例之前判断。
+# 生命周期自检（--hold-after-start）不测量吞吐。
 resource_gate() {
   local avail_kb load1
   avail_kb="$(df -Pk "$ROOT" | awk 'NR==2 { print $4 }')"
@@ -157,6 +158,10 @@ cleanup() {
 }
 trap cleanup EXIT INT TERM
 
+if [[ "$HOLD_AFTER_START" -eq 0 ]]; then
+  resource_gate
+fi
+
 TS="$(date +%Y%m%d-%H%M%S)"
 RESULTS="$HERE/results/pgbench-$TS"
 mkdir -p "$RESULTS"
@@ -191,8 +196,6 @@ if [[ "$HOLD_AFTER_START" -eq 1 ]]; then
   sleep 3600
 fi
 
-resource_gate
-
 DSN="$("$ROOT/scripts/svc.sh" db dsn "$NAME" --reveal)"
 
 psql "$DSN" -v ON_ERROR_STOP=1 -c "SELECT version()" >"$RESULTS/pg-version.txt"