Update doc for bench playbook

doc/bench.md

@@ -52,7 +52,7 @@ bench_host_api_key: <MediaServer master API key>
 ```
 # ansible-playbook -i inventories/<your inventory> playbooks/bench.yml
 ```
-
+This playbook will install everything you need : locust server, locust workers, and benchmarker.
 
 ## Start the benchmarker
 
@@ -71,3 +71,28 @@ Go with your browser on `http://<bench_server>:8089`. You might have to use an S
 Set the number of viewers and the hatch rate (usually the hatch rate is 10% of the viewers count) and start the bench.
 
 Watch for `/var/log/access.log` (warning: this file is buffered by default, you can disable buffer in `nginx.conf`).
+
+## Restart Locust server & workers
+
+If you need to restart both server & workers, you can launch the playbook with the tag `prepare-bench`
+```
+# ansible-playbook -i inventories/<your inventory> playbooks/bench.yml -t prepare-bench
+```
+
+## /!\ EXPERIMENTAL : Elasticsearch + Kibana + metricbeat
+
+Kibana + metricbeat allows you to monitor and display statistics about the infrastructure. This feature is experimental.
+
+First you need to add a new group to your hosts files :
+```
+[elastic]
+elastic1.test.com
+```
+
+Then launch the playbook with the tag `monbench` wich will install both the elastic suite on the elastic host, and metricbeat on mediaserver & postgres servers.
+
+```
+# ansible-playbook -i inventories/<your inventory> playbooks/bench.yml -t monbench
+```
+
+Your kibana instance will be accessible at `http://elastic1.test.com:5601`