Calculate IntervalDelta correctly if there are no changes to the counter

metrics/counter.go

@@ -34,7 +34,7 @@ func (c *Counter) Value() int64 {
 
 // IntervalValue allows to get last interval value for counter.
 func (c *Counter) IntervalValue() int64 {
-	if c.lastIntervalDelta == 0 {
+	if c.lastIntervalValue == 0 {
 		return c.Value()
 	}
 	return atomic.LoadInt64(&c.lastIntervalDelta)

metrics/counter_test.go

@@ -19,4 +19,10 @@ func TestCounter(t *testing.T) {
 	cnt.Inc()
 	assert.Equal(t, int64(1501), cnt.Value())
 	assert.Equal(t, int64(1500), cnt.IntervalValue())
+	cnt.UpdateDelta()
+	assert.Equal(t, int64(1501), cnt.Value())
+	assert.Equal(t, int64(1), cnt.IntervalValue())
+	cnt.UpdateDelta()
+	assert.Equal(t, int64(1501), cnt.Value())
+	assert.Equal(t, int64(0), cnt.IntervalValue())
 }