Use log_append for upstream details

src/docs/markdown/caddyfile/directives/log_append.md

@@ -37,3 +37,22 @@ example.com {
 	}
 }
 ```
+
+Display in the logs, which reverse proxy upstream was effectively used (either `node1:80`, `node2:80` or `node3:80`) and
+the time spent proxying to the upstream in milliseconds as well as how long it took the proxy upstream to write the response header:
+
+```caddy
+example.com {
+	log
+
+	handle {
+		reverse_proxy node1:80 node2:80 node3:80 {
+			header_up Host {upstream_hostport}
+			lb_policy random_choose 2 
+        }
+		log_append upstream-host "{http.reverse_proxy.upstream.host}"
+		log_append upstream-duration-ms "{http.reverse_proxy.upstream.duration_ms}"
+		log_append upstream-latency-ms "{http.reverse_proxy.upstream.latency_ms}"
+	}
+}
+```