Fix Prometheus exporter

Hopefully by explicitly setting type and help messages the node exporter
will be able to merge values for the same metric when spread across
multiple files.

debian/changelog

+runcron (1.1.1) unstable; urgency=medium
+
+  * Fix Prometheus exporter.
+
+ -- ale <ale@incal.net>  Sat, 09 Feb 2019 10:55:59 +0000
+
 runcron (1.1) unstable; urgency=medium
 
   * Fix version number.

runcron.c

@@ -466,6 +466,14 @@ int run(char **argv) {
   return exit_status;
 }
 
+static void print_prom_metric(FILE *fp, const char *name, const char *help,
+                              const char *type, long value) {
+  fprintf(fp, "# HELP %s %s\n"
+              "# TYPE %s %s\n"
+              "%s{cronjob=\"%s\"} %ld\n",
+          name, help, name, type, name, job_name, value);
+}
+
 int export_metrics(int exit_status) {
   struct stat sb;
   char *tmp_file = NULL, *metrics_file = NULL;
@@ -478,12 +486,12 @@ int export_metrics(int exit_status) {
 
   if (asprintf(&tmp_file, "%s/.cron-%s.prom.tmp~", node_exporter_dir,
                job_name) < 0) {
-    fprintf(stderr, "Error: could not allocate memory for metrics\n");
+    fprintf(stderr, "Error: could not allocate memory\n");
     return -1;
   }
   if (asprintf(&metrics_file, "%s/cron-%s.prom", node_exporter_dir, job_name) <
       0) {
-    fprintf(stderr, "Error: could not allocate memory for metrics\n");
+    fprintf(stderr, "Error: could not allocate memory\n");
     return -1;
   }
 
@@ -496,14 +504,17 @@ int export_metrics(int exit_status) {
             strerror(errno));
     return -1;
   }
-  fprintf(fp, "cronjob_status{cronjob=\"%s\"} %d\n", job_name,
+  print_prom_metric(fp, "cronjob_status", "Status of the cron job.", "gauge",
                     (exit_status == 0) ? 1 : 0);
-  fprintf(fp, "cronjob_last_start{cronjob=\"%s\"} %ld\n", job_name, start_time);
-  fprintf(fp, "cronjob_last_end{cronjob=\"%s\"} %ld\n", job_name, end_time);
-  fprintf(fp, "cronjob_runtime_seconds{cronjob=\"%s\"} %d\n", job_name,
-          elapsed);
+  print_prom_metric(fp, "cronjob_last_start", "Last cron job start time.",
+                    "gauge", start_time);
+  print_prom_metric(fp, "cronjob_last_end", "Last cron job end time.", "gauge",
+                    end_time);
+  print_prom_metric(fp, "cronjob_runtime_seconds",
+                    "Duration of the last cron job run.", "gauge", elapsed);
   if (exit_status == 0) {
-    fprintf(fp, "cronjob_last_success{cronjob=\"%s\"} %ld\n", job_name,
+    print_prom_metric(fp, "cronjob_last_success",
+                      "End time of the last successful cron job.", "gauge",
                       end_time);
   }
   fclose(fp);