Release note related to envoy #2787 (#567)

Signed-off-by: Gary Brown <gary@brownuk.com>

docs/root/intro/version_history.rst

@@ -11,6 +11,9 @@ Version history
 * admin: added ``/stats/prometheus`` as an alternative endpoint for getting stats in prometheus format.
 * logger: all :ref:`logging levels <operations_admin_interface_logging>` can be configured
   at run-time: trace debug info warning error critical.
+* tracing: the sampling decision is now delegated to the tracers, allowing the tracer to decide when and if
+  to use it. For example, if the :ref:`x-b3-sampled <config_http_conn_man_headers_x-b3-sampled>` header
+  is supplied with the client request, its value will override any sampling decision made by the Envoy proxy.
 
 1.6.0
 =====