Allow regex to keep for promql/aggregate

Hey! Can you please allow regex to be used for "keep"? My prometheus uses a webhook to send alerts to corporate monitoring, it expects to receive alerts with at least one label from the set.

For example:
```
rule {
match {
kind = "alerting"
}

aggregate ".+" {
keep = ["lable1|lable2"]
}
}
```

more information:

Is there a way to check label by regex in the resulting vector, like promql/series (api request)? (after aggregating and adding additional labels)

I whant find alerts WITHOUT special_label
```
#situation 1:

- name: ""
rules:
- alert: AlertName
expr:
labels:
special_label: ...

#situation 2:

- name: ""
rules:
- alert: AlertName
expr: sum by (special_label)...
```