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.

src/ngx_http_ipng_stats_module.c

@@ -194,6 +194,7 @@ typedef struct {
     struct sockaddr_in  logtail_udp_addr;  /* destination address */
     size_t              logtail_buf_size;   /* per-worker buffer, default 64k */
     ngx_msec_t          logtail_flush;     /* max flush interval, default 1s */
+    ngx_uint_t          logtail_if_index;  /* if=$var: variable index, 0=none */
 } ngx_http_ipng_stats_main_conf_t;
 
 
@@ -908,6 +909,27 @@ ngx_http_ipng_stats_logtail(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
             }
             continue;
         }
+        if (value[i].len > 3
+            && ngx_strncmp(value[i].data, "if=", 3) == 0)
+        {
+            ngx_str_t var_name = { value[i].len - 3, value[i].data + 3 };
+            if (var_name.len < 2 || var_name.data[0] != '$') {
+                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
+                    "ipng_stats_logtail: if= requires a $variable, "
+                    "got \"%V\"", &value[i]);
+                return NGX_CONF_ERROR;
+            }
+            var_name.data++;
+            var_name.len--;
+            imcf->logtail_if_index = ngx_http_get_variable_index(cf,
+                                                                  &var_name);
+            if (imcf->logtail_if_index == (ngx_uint_t) NGX_ERROR) {
+                return NGX_CONF_ERROR;
+            }
+            /* Shift from 0-based to 1-based so 0 means "not set". */
+            imcf->logtail_if_index++;
+            continue;
+        }
         ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
             "ipng_stats_logtail: unknown parameter \"%V\"", &value[i]);
         return NGX_CONF_ERROR;
@@ -1720,6 +1742,19 @@ ngx_http_ipng_stats_logtail_write(ngx_http_request_t *r,
         return;
     }
 
+    /* if=$variable: skip this request when the variable is empty or "0". */
+    if (imcf->logtail_if_index) {
+        ngx_http_variable_value_t  *val;
+
+        val = ngx_http_get_indexed_variable(r, imcf->logtail_if_index - 1);
+        if (val == NULL || val->not_found
+            || val->len == 0
+            || (val->len == 1 && val->data[0] == '0'))
+        {
+            return;
+        }
+    }
+
     ops  = imcf->logtail_fmt->ops->elts;
     nops = imcf->logtail_fmt->ops->nelts;