MINOR: promex: Add global and proxies description as labels to all metrics

While the global description is exposed, when defined, in a dedicated
metric, it is not possible to dump the description defined in a
frontend/listen/backend sections. So, thanks to this patch, it is now
possible to dump it as a label of all metrics of the corresponding
section. To do so, "desc-labels" parameter must be provided on the URL:

    /metrics?desc-labels

When this parameter is set, if a description is provided in a section,
including the global one, the "desc" label will be added to all metrics of
this section. For instance:

  haproxy_frontend_current_sessions{proxy="front-http",desc="..."} 1

Note that servers metrics inherit the description of their backend/listen
section.

This patch should solve the issue #1531.

Author: capflam

addons/promex/README

@@ -91,6 +91,18 @@ name must be preceded by a minus character ('-'). Here are examples:
   # Only dump frontends, backends and servers status
   /metrics?metrics=haproxy_frontend_status,haproxy_backend_status,haproxy_server_status
 
+* Add section description as label for all metrics
+
+It is possible to set a description in global and proxy sections, via the
+"description" directive. The global descrption is exposed if it is define via
+the "haproxy_process_description" metric. But the descriptions provided in proxy
+sections are not dumped. However, it is possible to add it as a label for all
+metrics of the corresponding section, including the global one. To do so,
+"desc-labels" parameter must be set:
+
+  /metrics?desc-labels
+
+  / metrics?scope=frontend&desc-labels
 
 * Dump extra counters
 

addons/promex/include/promex/promex.h

@@ -47,6 +47,7 @@
 #define PROMEX_FL_NO_MAINT_SRV      0x00002000
 #define PROMEX_FL_EXTRA_COUNTERS    0x00004000
 #define PROMEX_FL_INC_METRIC_BY_DEFAULT 0x00008000
+#define PROMEX_FL_DESC_LABELS       0x00010000
 
 #define PROMEX_FL_SCOPE_ALL (PROMEX_FL_SCOPE_GLOBAL | PROMEX_FL_SCOPE_FRONT | \
 			     PROMEX_FL_SCOPE_LI | PROMEX_FL_SCOPE_BACK | \