Differentiate errors when the DNS resolver is unreachable versus a DNS error during validation

[mcpherrinm]

During a recent outage, the DNS resolver was unreachable.

In boulder's metrics, it isn't possible to distinguish an error reaching the resolver versus one caused by validation.

I'd like to be able to distinguish those cases, as it's an important split between an internal and external problem.

I'll open an LLM PR which implements what I'd like, though I haven't put much effort into it, so please feel free to just close it and handle differently if desired.

[mcpherrinm]

After discussion with @aarongable, I think it would be good to also split validation metrics based on the type of identifier (dns, IP), because while the challenge types are shared, there are some failure modes unique to each.