Add variables.

README.md

@@ -51,7 +51,7 @@ Dumping all the requests could be useful even in non-distributed environment.
 http {
     server {
         location / {
-            otel_trace on;
+            otel_trace $otel_parent_sampled;
             otel_trace_context propagate;
 
             proxy_pass http://backend;
@@ -60,6 +60,33 @@ http {
 }
 ```
 
+### Ratio-based Tracing
+
+```nginx
+http {
+    # trace 10% of requests
+    split_clients $otel_trace_id $ratio_sampler {
+        10%     on;
+        *       off;
+    }
+
+    # or we can trace 10% of user sessions
+    split_clients $cookie_sessionid $session_sampler {
+        10%     on;
+        *       off;
+    }
+
+    server {
+        location / {
+            otel_trace $ratio_sampler;
+            otel_trace_context inject;
+
+            proxy_pass http://backend;
+        }
+    }
+}
+```
+
 ## How to Use
 
 ### Directives
@@ -111,6 +138,44 @@ Maximum number of spans to be sent in one batch per worker. Detault is 512.
 
 Maximum number of pending batches per worker, over the limit spans are dropped. Default is 4.
 
+### Variables
+
+`$otel_trace_id` - trace id.
+
+`$otel_span_id` - current span id.
+
+`$otel_parent_id` - parent span id.
+
+`$otel_parent_sampled` - `sampled` flag of parent span, `1`/`0`.
+
+### Default span [attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/semantic_conventions/http.md)
+
+`http.method`
+
+`http.target`
+
+`http.route`
+
+`http.scheme`
+
+`http.flavor`
+
+`http.user_agent`
+
+`http.request_content_length`
+
+`http.response_content_length`
+
+`http.status_code`
+
+`net.host.name`
+
+`net.host.port`
+
+`net.sock.peer.addr`
+
+`net.sock.peer.port`
+
 ## License
 
 [Apache License, Version 2.0](https://github.com/nginxinc/nginx-otel/blob/main/LICENSE)