cmd/roachtest: adjust disk-stalled roachtests TPS calculation

Previously, the post-stall TPS calculation included the time that the node was
stalled but before the stall triggered the node's exit. During this period,
overall TPS drops until the gray failure is converted into a hard failure. This
commit adjusts the post-stall TPS calculation to exclude the stalled time when
TPS is expected to tank.

Epic: None
Informs: #97705.
Release note: None

pkg/cmd/roachtest/tests/disk_stall.go

@@ -208,14 +208,12 @@ func runDiskStalledDetection(
 	}
 
 	// Let the workload continue after the stall.
-	{
-		workloadPauseDur := 10*time.Minute - timeutil.Since(workloadStartAt)
-		t.Status("letting workload continue for ", workloadPauseDur, " with n1 stalled")
-		select {
-		case <-ctx.Done():
-			t.Fatal(ctx.Err())
-		case <-time.After(workloadPauseDur):
-		}
+	workloadAfterDur := 10*time.Minute - timeutil.Since(workloadStartAt)
+	t.Status("letting workload continue for ", workloadAfterDur, " with n1 stalled")
+	select {
+	case <-ctx.Done():
+		t.Fatal(ctx.Err())
+	case <-time.After(workloadAfterDur):
 	}
 
 	{
@@ -226,7 +224,7 @@ func runDiskStalledDetection(
 		cum := response.Results[0].Datapoints
 		totalTxnsPostStall := cum[len(cum)-1].Value - totalTxnsPreStall
 		preStallTPS := totalTxnsPreStall / stallAt.Sub(workloadStartAt).Seconds()
-		postStallTPS := totalTxnsPostStall / now.Sub(stallAt).Seconds()
+		postStallTPS := totalTxnsPostStall / workloadAfterDur.Seconds()
 		t.L().PrintfCtx(ctx, "%.2f total transactions committed after stall\n", totalTxnsPostStall)
 		t.L().PrintfCtx(ctx, "pre-stall tps: %.2f, post-stall tps: %.2f\n", preStallTPS, postStallTPS)
 		if postStallTPS < preStallTPS/2 {