### Describe Your Changes
* check if `lastValue` was seen at least twice with different
timestamps. Otherwise, the difference between last timestamp and
previous timestamp could be `0` and will result into `NaN` calculation
* check if there items left in lastValue map after staleness cleanup.
Otherwise, `rate_avg` could have produce `NaN` result.
### Checklist
The following checks are **mandatory**:
- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
---------
Signed-off-by: hagen1778 <roman@victoriametrics.com>
(cherry picked from commit 51d19485bb)
5df50e5645