improve resolution of server error counters

per comment https://github.com/katzenpost/katzenpost/pull/301#discussion_r1275144769

```
threebithacker Jul 26, 2023

These two cases of instrument.KaetzchenRequestsDropped(1) are really dropped responses aren't they? It feels like we might want a separate counter for that. If this is prometheus counters, it would probably be helpful to service operators if we could label them with the kaetchzen name so it's possible to narrow down which service is generating faulty replies.

```

Start with the linked pull request discussion and locate the two instrument.KaetzchenRequestsDropped(1) calls mentioned there. Determine whether they represent dropped responses, then define separate counters and kaetzchen-name labels so operators can identify the service generating faulty replies; no test or file path is provided in the issue.

Written by the indexing model from the issue text.