fix empty query result issue cause by vector(0)

[viperstars]

vector(0) in some recording rule could cause a empty query result error. For example:

      (
        (
          # too slow
          sum by (cluster) (rate(apiserver_request_duration_seconds_count{job="kube-apiserver",verb=~"LIST|GET"}[1d]))
          -
          (
            (
              sum by (cluster) (rate(apiserver_request_duration_seconds_bucket{job="kube-apiserver",verb=~"LIST|GET",scope=~"resource|",le="0.1"}[1d]))
              or
              vector(0)
            )
            +
            sum by (cluster) (rate(apiserver_request_duration_seconds_bucket{job="kube-apiserver",verb=~"LIST|GET",scope="namespace",le="0.5"}[1d]))
            +
            sum by (cluster) (rate(apiserver_request_duration_seconds_bucket{job="kube-apiserver",verb=~"LIST|GET",scope="cluster",le="5"}[1d]))
          )
        )
        +
        # errors
        sum by (cluster) (rate(apiserver_request_total{job="kube-apiserver",verb=~"LIST|GET",code=~"5.."}[1d]))
      )
      /
      sum by (cluster) (rate(apiserver_request_total{job="kube-apiserver",verb=~"LIST|GET"}[1d]))

If sum by (cluster) (rate(apiserver_request_duration_seconds_bucket{job="kube-apiserver",verb=~"LIST|GET",scope=~"resource|",le="0.1"}[1d])) returns an empty query result, sum by (cluster) (rate(apiserver_request_duration_seconds_bucket{job="kube-apiserver",verb=~"LIST|GET",scope=~"resource|",le="0.1"}[1d])) or vector(0) will return {} 0 which lost cluster tag and whole recoding rule returns a an empty query result.

The vector(0) should be replace by (min(apiserver_current_inflight_requests{job="kube-apiserver"}) by (cluster) * 0). The difference is the second clause will return zero with cluster tags.

[brancz]

I'm not sure the query still works as expected with these changes, but I agree we need to find a solution to the cluster label and vector(0) problem.

@metalmatze ideas?

[metalmatze]

Yep, agreed with brancz. I guess we need to fix it if it breaks but I'm not sure it'll work the way it works right now with approach.