metrics: improve TestTimerFunc (ethereum#20818)

The test failed due to what appears to be fluctuations in time.Sleep, which is
not the actual method under test. This change modifies it so we compare the
metered Max to the actual time instead of the desired time.

metrics/timer_test.go

@@ -45,10 +45,23 @@ func TestTimerStop(t *testing.T) {
 }
 
 func TestTimerFunc(t *testing.T) {
-	tm := NewTimer()
-	tm.Time(func() { time.Sleep(50e6) })
-	if max := tm.Max(); 35e6 > max || max > 145e6 {
-		t.Errorf("tm.Max(): 35e6 > %v || %v > 145e6\n", max, max)
+	var (
+		tm         = NewTimer()
+		testStart  = time.Now()
+		actualTime time.Duration
+	)
+	tm.Time(func() {
+		time.Sleep(50 * time.Millisecond)
+		actualTime = time.Since(testStart)
+	})
+	var (
+		drift    = time.Millisecond * 2
+		measured = time.Duration(tm.Max())
+		ceil     = actualTime + drift
+		floor    = actualTime - drift
+	)
+	if measured > ceil || measured < floor {
+		t.Errorf("tm.Max(): %v > %v || %v > %v\n", measured, ceil, measured, floor)
 	}
 }