chore(spanner): use time.Since instead of time.Now().Sub (#10096)

time.Since has few optimizations to get monotonic time, also it's shorter and clearer.

spanner/client_test.go

@@ -1487,10 +1487,10 @@ func TestClient_ReadOnlyTransaction_WhenMultipleOperations_SessionLastUseTimeSho
 		t.Fatalf("Session lastUseTime times should not be equal")
 	}
 
-	if (time.Now().Sub(sessionPrevLastUseTime)).Milliseconds() < 400 {
+	if time.Since(sessionPrevLastUseTime).Milliseconds() < 400 {
 		t.Fatalf("Expected session to be checkedout for more than 400 milliseconds")
 	}
-	if (time.Now().Sub(sessionCheckoutTime)).Milliseconds() < 400 {
+	if time.Since(sessionCheckoutTime).Milliseconds() < 400 {
 		t.Fatalf("Expected session to be checkedout for more than 400 milliseconds")
 	}
 	// force run task to clean up unexpected long-running sessions whose lastUseTime >= 3sec.
@@ -2000,10 +2000,10 @@ func TestClient_ReadWriteTransaction_WhenMultipleOperations_SessionLastUseTimeSh
 			t.Fatalf("Session lastUseTime times should not be equal")
 		}
 
-		if (time.Now().Sub(sessionPrevLastUseTime)).Milliseconds() < 400 {
+		if time.Since(sessionPrevLastUseTime).Milliseconds() < 400 {
 			t.Fatalf("Expected session to be checkedout for more than 400 milliseconds")
 		}
-		if (time.Now().Sub(sessionCheckoutTime)).Milliseconds() < 400 {
+		if time.Since(sessionCheckoutTime).Milliseconds() < 400 {
 			t.Fatalf("Expected session to be checkedout for more than 400 milliseconds")
 		}
 		// force run task to clean up unexpected long-running sessions whose lastUseTime >= 3sec.

spanner/integration_test.go

@@ -586,7 +586,7 @@ func TestIntegration_SingleUse(t *testing.T) {
 	}
 	// Calculate time difference between Cloud Spanner server and localhost to
 	// use to determine the exact staleness value to use.
-	timeDiff := maxDuration(time.Now().Sub(writes[0].ts), 0)
+	timeDiff := maxDuration(time.Since(writes[0].ts), 0)
 
 	// Test reading rows with different timestamp bounds.
 	for i, test := range []struct {
@@ -649,7 +649,7 @@ func TestIntegration_SingleUse(t *testing.T) {
 			skipForPG: true,
 			want:      nil,
 			// Specify a staleness which should be already before this test.
-			tb: ExactStaleness(time.Now().Sub(writes[0].ts) + timeDiff + 30*time.Second),
+			tb: ExactStaleness(time.Since(writes[0].ts) + timeDiff + 30*time.Second),
 			checkTs: func(ts time.Time) error {
 				if !ts.Before(writes[0].ts) {
 					return fmt.Errorf("read got timestamp %v, want it to be earlier than %v", ts, writes[0].ts)

spanner/session.go

@@ -746,7 +746,7 @@ func (p *sessionPool) getLongRunningSessionsLocked() []*sessionHandle {
 				element = element.Next()
 				continue
 			}
-			diff := time.Now().Sub(sh.lastUseTime)
+			diff := time.Since(sh.lastUseTime)
 			if !sh.eligibleForLongRunning && diff.Seconds() >= p.idleTimeThreshold.Seconds() {
 				if (p.ActionOnInactiveTransaction == Warn || p.ActionOnInactiveTransaction == WarnAndClose) && !sh.isSessionLeakLogged {
 					if p.ActionOnInactiveTransaction == Warn {