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_conf_validation.c

@@ -344,6 +344,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_http_members[] = {
         .type       = NXT_CONF_VLDT_OBJECT,
         .validator  = nxt_conf_vldt_object,
         .u.members  = nxt_conf_vldt_static_members,
+    }, {
+        .name       = nxt_string("log_route"),
+        .type       = NXT_CONF_VLDT_BOOLEAN,
     },
 
     NXT_CONF_VLDT_END