On templates validation stage vmalert does not acutally send queries, so for complex
chained expression validation may fail. To avoid this, we add a blank sample in response
so validation can pass successfully. Later, during the rule execution, stub will be replaced
with real `query` function.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/989
The commit adds a support for template function `query`,
`first` and `value`. The function `query` executes
a MetricsQL query for active alerts. In vmalert we
update templates on every evaluation for active alerts
to keep them up to date. With `query` func it may become
a perf issue since it will fire a query on every execution.
We should keep it in mind for now.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/539
