docs: mention tail sampling

docs/docs/configuration/sampling-tracetest-spans.mdx

@@ -91,6 +91,22 @@ service:
       exporters: [otlp/jaeger]
 ```
 
+### Tail sampling approach
+Before December 2023, we were suggesting people to use tail sampling to filter the traces generated by tracetests.
+However, the new `filter` capabilities are better for performance than tail sampling as it
+requires less memory to decide if a trace should be sampled or not. But even with those arguments you still want to use the
+tail sampling approach, this is the processor you can use:
+
+```yaml
+processors:
+  tail_sampling:
+    decision_wait: 5s
+    policies:
+      - name: tracetest-spans
+        type: trace_state
+        trace_state: { key: tracetest, values: ["true"] }
+```
+
 With this configuration, you will still get 5% of all your traces, but you will also ensure that all your test traces are collected and sent to
 Jaeger.