MINOR: contrib/prometheus-exporter: export build_info

commit c55a626217 ("MINOR: contrib/prometheus-exporter: Add
missing global and per-server metrics") is renaming two metrics between
v2.2 and v2.3:
  server_idle_connections_current
  server_idle_connections_limit

It is breaking some tools which are making use of those metrics while
supporting several haproxy versions. This build_info will permit tools
which make use of metrics to be able to match the haproxy version and
change the list of expected metrics. This was possible using the haproxy
stats socket but not with prometheus export.

This patch follows prometheus best pratices to export specific software
informations. It is adding a new field `build_info` so we can extend it
to other parameters if needed in the future.

example output:
  # HELP haproxy_process_build_info HAProxy build info.
  # TYPE haproxy_process_build_info gauge
  haproxy_process_build_info{version="2.4-dev5-2e1a3f-5"} 1

Even though it is not a bugfix, this patch will make more sense when
backported up to >= 2.0

Signed-off-by: William Dauchy <wdauchy@gmail.com>

contrib/prometheus-exporter/service-prometheus.c

@@ -38,6 +38,7 @@
 #include <haproxy/stream.h>
 #include <haproxy/stream_interface.h>
 #include <haproxy/task.h>
+#include <haproxy/version.h>
 
 /* Prometheus exporter applet states (appctx->st0) */
 enum {
@@ -82,12 +83,17 @@ enum {
  */
 #define PROMEX_MAX_METRIC_LENGTH 512
 
+/* Some labels for build_info */
+#define PROMEX_VERSION_LABEL "version=\"" HAPROXY_VERSION "\""
+#define PROMEX_BUILDINFO_LABEL PROMEX_VERSION_LABEL
+
 /* Matrix used to dump global metrics. Each metric points to the next one to be
  * processed or 0 to stop the dump. */
 const int promex_global_metrics[INF_TOTAL_FIELDS] = {
-	[INF_NAME]                           = INF_NBTHREAD,
+	[INF_NAME]                           = INF_BUILD_INFO,
 	[INF_VERSION]                        = 0,
 	[INF_RELEASE_DATE]                   = 0,
+	[INF_BUILD_INFO]                     = INF_NBTHREAD,
 	[INF_NBTHREAD]                       = INF_NBPROC,
 	[INF_NBPROC]                         = INF_PROCESS_NUM,
 	[INF_PROCESS_NUM]                    = INF_UPTIME_SEC,
@@ -469,6 +475,7 @@ const struct ist promex_inf_metric_names[INF_TOTAL_FIELDS] = {
 	[INF_NAME]                           = IST("name"),
 	[INF_VERSION]                        = IST("version"),
 	[INF_RELEASE_DATE]                   = IST("release_date"),
+	[INF_BUILD_INFO]                     = IST("build_info"),
 	[INF_NBTHREAD]                       = IST("nbthread"),
 	[INF_NBPROC]                         = IST("nbproc"),
 	[INF_PROCESS_NUM]                    = IST("relative_process_id"),
@@ -641,6 +648,7 @@ const struct ist promex_inf_metric_desc[INF_TOTAL_FIELDS] = {
 	[INF_NAME]                           = IST("Product name."),
 	[INF_VERSION]                        = IST("HAProxy version."),
 	[INF_RELEASE_DATE]                   = IST("HAProxy release date."),
+	[INF_BUILD_INFO]                     = IST("HAProxy build info."),
 	[INF_NBTHREAD]                       = IST("Configured number of threads."),
 	[INF_NBPROC]                         = IST("Configured number of processes."),
 	[INF_PROCESS_NUM]                    = IST("Relative process id, starting at 1."),
@@ -813,6 +821,7 @@ const struct ist promex_inf_metric_labels[INF_TOTAL_FIELDS] = {
 	[INF_NAME]                           = IST(""),
 	[INF_VERSION]                        = IST(""),
 	[INF_RELEASE_DATE]                   = IST(""),
+	[INF_BUILD_INFO]                     = IST(PROMEX_BUILDINFO_LABEL),
 	[INF_NBTHREAD]                       = IST(""),
 	[INF_NBPROC]                         = IST(""),
 	[INF_PROCESS_NUM]                    = IST(""),
@@ -978,6 +987,7 @@ const struct ist promex_inf_metric_types[INF_TOTAL_FIELDS] = {
 	[INF_NAME]                           = IST("untyped"),
 	[INF_VERSION]                        = IST("untyped"),
 	[INF_RELEASE_DATE]                   = IST("untyped"),
+	[INF_BUILD_INFO]                     = IST("gauge"),
 	[INF_NBTHREAD]                       = IST("gauge"),
 	[INF_NBPROC]                         = IST("gauge"),
 	[INF_PROCESS_NUM]                    = IST("gauge"),
@@ -1327,6 +1337,9 @@ static int promex_dump_global_metrics(struct appctx *appctx, struct htx *htx)
 #endif
 	while (appctx->st2 && appctx->st2 < INF_TOTAL_FIELDS) {
 		switch (appctx->st2) {
+			case INF_BUILD_INFO:
+				metric = mkf_u32(FN_GAUGE, 1);
+				break;
 			case INF_NBTHREAD:
 				metric = mkf_u32(FO_CONFIG|FS_SERVICE, global.nbthread);
 				break;

include/haproxy/stats-t.h

@@ -323,6 +323,7 @@ enum info_field {
 	INF_BYTES_OUT_RATE,
 	INF_DEBUG_COMMANDS_ISSUED,
 	INF_CUM_LOG_MSGS,
+	INF_BUILD_INFO,
 
 	/* must always be the last one */
 	INF_TOTAL_FIELDS