A client that goes away mid-request cancels the context; a handler then
fails on context.Canceled and writes a 500 nobody receives. The request
log now records it as 499 (client closed request) so a grep for 5xx shows
real server errors only.
- one Info record "http request" per request with method, matched route
pattern, status and duration_ms; path without the query only when no
route matched
- the matched route records its pattern through a request-scoped entry,
so locale prefix rewrites do not hide it
- logs through the published *slog.Logger or slog.Default; off by default
- surf README, logging and configuration docs describe the key
