Add logtail if=$variable filtering and update log format examples

- ipng_stats_logtail now accepts an optional if=$variable parameter
  that suppresses log lines when the variable is empty or "0",
  following the same semantics as nginx's access_log if=. The
  condition is checked before format rendering for zero overhead on
  filtered requests. Filtered requests are still counted by stats.
- Log format examples updated to include $scheme for http/https
  visibility, and renamed to ipng_stats_logtail to match production.
- Robot test added for the if= filter (19 tests, 19 pass).
- FR-8.5 added to design doc for the if= semantics.

tests/01-module/lab/server/nginx.conf

@@ -19,10 +19,15 @@ http {
     access_log /var/log/nginx/access.log tagged;
 
     # Global logtail — fires for ALL requests regardless of server block.
-    log_format logtail '$host\t$remote_addr\t$ipng_source_tag\t$server_addr\t'
-                       '$request_method\t$request_uri\t$status\t$body_bytes_sent\t'
-                       '$request_time';
-    ipng_stats_logtail logtail udp://127.0.0.1:9514 buffer=4k flush=500ms;
+    # The if= condition suppresses /notfound from the logtail stream.
+    map $request_uri $logtail_enabled {
+        ~^/notfound  0;
+        default      1;
+    }
+    log_format ipng_stats_logtail '$host\t$remote_addr\t$request_method\t$request_uri\t'
+                                  '$status\t$body_bytes_sent\t'
+                                  '$ipng_source_tag\t$server_addr\t$scheme';
+    ipng_stats_logtail ipng_stats_logtail udp://127.0.0.1:9514 buffer=4k flush=500ms if=$logtail_enabled;
 
     server {
         # Mgmt-only listener for direct traffic (tagged "direct").