Alerting: Fix duration calculation when testing a rule (#56616) (#56705)

When testing a rule within the legacy alerting the `timeMs` field could
sometimes show negative (and/or wrongly calculated) durations. This happens if the alert evaluation crosses a second boundary. This change uses the full timestamp to compute the eval duration.

(cherry picked from commit 95b9fa3346)

Co-authored-by: Jorge Luis Betancourt <jorge-luis.betancourt@trivago.com>

pkg/services/alerting/eval_context.go

@@ -107,7 +107,7 @@ func (c *EvalContext) shouldUpdateAlertState() bool {
 
 // GetDurationMs returns the duration of the alert evaluation.
 func (c *EvalContext) GetDurationMs() float64 {
-	return float64(c.EndTime.Nanosecond()-c.StartTime.Nanosecond()) / float64(1000000)
+	return float64(c.EndTime.Sub(c.StartTime).Nanoseconds()) / float64(time.Millisecond)
 }
 
 // GetNotificationTitle returns the title of the alert rule including alert state.

pkg/services/alerting/eval_context_test.go

@@ -404,3 +404,18 @@ func TestEvaluateNotificationTemplateFields(t *testing.T) {
 		})
 	}
 }
+
+func TestGetDurationFromEvalContext(t *testing.T) {
+	startTime, err := time.Parse("2006-01-02 15:04:05.999999999 -0700 MST", "2022-10-03 11:33:14.438803 +0200 CEST")
+	require.NoError(t, err)
+
+	endTime, err := time.Parse("2006-01-02 15:04:05.999999999 -0700 MST", "2022-10-03 11:33:15.291075 +0200 CEST")
+	require.NoError(t, err)
+
+	evalContext := EvalContext{
+		StartTime: startTime,
+		EndTime:   endTime,
+	}
+
+	assert.Equal(t, float64(852.272), evalContext.GetDurationMs())
+}