Update tracing documentation to include tracing-headers

docs/operate/monitoring/tracing.mdx

@@ -52,6 +52,18 @@ For example, locally with Docker:
 
 </CH.Code>
 
+<details className={"grey-details"}>
+    <summary>Specifying additional tracing headers</summary>
+
+    You can specify additional headers to be sent with the trace data by setting the `tracing-headers` configuration entry.
+    For example, to specify an `authorization` header add the following snippet to the [configuration file](/operate/configuration/server/#configuration-file):
+
+    ```toml
+    [tracing-headers]
+    authorization = "Bearer some-auth-token"
+    ```
+</details>
+
 If you now spin up your services and send requests to them, you will see the traces appear in the Jaeger UI at http://localhost:16686:
 
 <img src="/img/tracing_tour.png" width={"60%"}/>