MEDIUM: contrib/prometheus-exporter: export base stick table stats

I saw some people falling back to unix socket to collect some data they
could not find in prometheus exporter. One of them is base info from
stick tables (used/size).
I do not plan to extend it more for now; keys are quite a mess to
handle.

This should resolve github issue #1008.

Signed-off-by: William Dauchy <wdauchy@gmail.com>
diff --git a/contrib/prometheus-exporter/README b/contrib/prometheus-exporter/README
index a859815..d882b09 100644
--- a/contrib/prometheus-exporter/README
+++ b/contrib/prometheus-exporter/README
@@ -72,6 +72,7 @@
   /metrics?scope=frontend&scope=backend # ==> Frontend and backend metrics will be exported
   /metrics?scope=*&scope=               # ==> no metrics will be exported
   /metrics?scope=&scope=global          # ==> global metrics will be exported
+  /metrics?scope=sticktable             # ==> stick tables metrics will be exported
 
 * How do I prevent my prometheus instance to explode?
 
@@ -320,3 +321,12 @@
 | haproxy_server_need_connections_current            |
 | haproxy_server_uweight                             |
 +----------------------------------------------------+
+
+* Stick table metrics
+
++----------------------------------------------------+
+|    Metric name                                     |
++----------------------------------------------------+
+| haproxy_sticktable_size                            |
+| haproxy_sticktable_used                            |
++----------------------------------------------------+