when trying to create an alert on high metric cardinality with the expression count by(__name__) ({__name__=~".+"}) > 50 I get the error: vector contains metrics with the same labelset after applying rule labels.
As the expression works when using it directly in prometheus, I wonder if there is an actual way to use it in an alert?
I think I found a solution for this issue, as I was trying it myself.
LT;DR
use this promQL expression for alerting on metric cardinality:
label_replace(count by(__name__) ({__name__=~".+"}), "name", "$1", "__name__", "(.+)") > 50
Long Version
The issue as stated in the Prometheus error message. After the metric vector is converted to a vector of the alert, no labels differ and therefore are duplicated. this means
vector A ( metric_a{label=test}, metric_b{label=test} )
is converted in
vector B ( alert_a{label=test}, alert_a{label=test})
and that is why you have duplicates
( caveat: that is at least my understanding) By adding a new label with the metric name itself, you create a unique label set.