metrics: show libp2p probe errors per user agent

[lidel]

coredns_forge_acme_libp2p_probe_total{result, agent} produces all errors under "unknown" agent name, because we cant read agentVersion via libp2p identify if libp2p connection failed:

NOTE: this is from internal testing period:

however, it should be technically possible to look at User-Agent header sent to the registration endpoint and still be able to tell that request comes from kubo or "other"