Update custom-prom.md

[ivankube]

cost-analyzer-frontend container does not have curl command:

~ kubectl exec -i -t --namespace kubecost \
  deployment/kubecost-cost-analyzer -c cost-analyzer-frontend -- curl
error: Internal error occurred: Internal error occurred: error executing command in container: failed to exec in container: failed to start exec "ddb09a068fea286a5f177cc0708c91c23e8d893dd9d585d9892fdfcb9da53456": OCI runtime exec failed: exec failed: unable to start container process: exec: "curl": executable file not found in $PATH: unknown

Proposed Changes

Replaced cost-analyzer-frontend with cost-model

[thomasvn]

Thank you for this update @ivankube! I've successfully tested this command in my own namespace running Kubecost:

kubectl exec -i -t --namespace thomasn-nightly \
  deployment/thomasn-nightly-cost-analyzer -c cost-model -- \
  curl "http://localhost:9003/prometheusQuery?query=node_total_hourly_cost"