[kube-prometheus-stack] Set a default commonName for prometheus cert-…

[maarous]

Summary

Set a default commonName in the Prometheus Operator cert-manager Certificate, equal to the Prometheus Operator service name.

Motivation

Some PKI certificate issuers require the commonName field to be set explicitly. Without it, the certificate request fails during signing.

For example, when using a Vault-backed issuer, the certificate request fails with the following error:

Warning Failed 44s cert-manager-certificates-issuing The certificate request has failed to complete and will be retried: Vault failed to sign certificate: failed to sign certificate by vault: Error making API request.
Code: 400. Errors:

• the common_name field is required, or must be provided in a CSR with "use_csr_common_name" set to true, unless "require_cn" is set to false

Setting the commonName to the Prometheus Operator service name ensures the certificate request satisfies these issuers and completes successfully.

[jkroepke]

LGTM