add dedicated counters for a few common 4xx and 5xx codes

Can we afford to add 5 or 15 counters to help troubleshoot these specific HTTP outcomes? These would be **in addition** to today's catch-all `http_4xx` and `http_5xx` counters.

Usually if I saw 4xx or 5xx errors in Nighthawk counters, I would just use `curl` against the server directly to see what's happening, but when using a custom transport socket, that's impossible.

If the resource cost is significant, we should prioritize the most common counters.

If we can afford 15:
- 400 Bad Request
- 401 Unauthorized
- 403 Forbidden
- 404 Not Found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout
- 429 Too Many Requests
- 500 Internal Server Error
- 501 Not Implemented
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Gateway Timeout
- 505 HTTP Version Not Supported

If we can only afford 5:
- 404 Not Found
- 500 Internal Server Error
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Gateway Timeout