HTTP: added route logging.

-  Configuration: added "/config/settings/http/log_route".

   Type: bool
   Default: false

   This adds configurability to the error log.  It allows enabling and
   disabling logs related to how the router performs selection of the
   routes.

-  HTTP: logging request line.

   Log level: [notice]

   The request line is essential to understand which logs correspond to
   which request when reading the logs.

-  HTTP: logging route that's been discarded.

   Log level: [info]

-  HTTP: logging route whose action is selected.

   Log level: [notice]

-  HTTP: logging when "fallback" action is taken.

   Log level: [notice]

Closes: <https://github.com/nginx/unit/issues/758>
Link: <https://github.com/nginx/unit/pull/824>
Link: <https://github.com/nginx/unit/pull/839>
Suggested-by: Timo Stark <t.stark@nginx.com>
Suggested-by: Mark L Wood-Patrick <mwoodpatrick@gmail.com>
Suggested-by: Liam Crilly <liam@nginx.com>
Tested-by: Liam Crilly <liam@nginx.com>
Acked-by: Artem Konev <a.konev@f5.com>
Cc: Andrew Clayton <a.clayton@nginx.com>
Cc: Andrei Zeliankou <zelenkov@nginx.com>
Reviewed-by: Zhidao Hong <z.hong@f5.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

src/nxt_router.c

@@ -1513,6 +1513,12 @@ static nxt_conf_map_t  nxt_router_http_conf[] = {
         NXT_CONF_MAP_INT8,
         offsetof(nxt_socket_conf_t, discard_unsafe_fields),
     },
+
+    {
+        nxt_string("log_route"),
+        NXT_CONF_MAP_INT8,
+        offsetof(nxt_socket_conf_t, log_route),
+    },
 };