fix: error log event metadata

[vreynolds]

Which problem is this PR solving?

• Refinery logs errors that it receives from libhoney (e.g. events are too large), but does not log any useful identifying information, making it very hard to find the source of the error
• internal slack thread

Short description of the changes

• aligned metadata attached to events with what we attach to error logs
• event_type and target used to be properties on libhoney events, but are now gone, so 🤷

how errors look before this change:

ERRO[0149] error when sending event                      api_host= dataset= error="got unexpected HTTP status 401: Unauthorized" event_type= status_code=401 target=

how error look after this change

ERRO[0219] error when sending event                      api_host="https://api-dogfood.honeycomb.io" dataset=year-java environment="huzzah env" error="got unexpected HTTP status 401: Unauthorized" status_code=401