BUG/MINOR: contrib/prometheus-exporter: Add HTX data block in one time

Since recent changes on the way HTX data blocks are added in an HTX message, we
must now be sure the prometheus service add its own blocks in one time. Indeed,
the function htx_add_data() may now decide to only copy a part of data. So
instead, we must call htx_add_data_atonce() instead.
diff --git a/contrib/prometheus-exporter/service-prometheus.c b/contrib/prometheus-exporter/service-prometheus.c
index 3071b8f..483e703 100644
--- a/contrib/prometheus-exporter/service-prometheus.c
+++ b/contrib/prometheus-exporter/service-prometheus.c
@@ -1409,7 +1409,7 @@
 	}
 
   end:
-	if (!htx_add_data(htx, out))
+	if (!htx_add_data_atonce(htx, out))
 		return -1; /* Unexpected and unrecoverable error */
 	channel_add_input(chn, out.len);
 	return ret;
@@ -1587,7 +1587,7 @@
 	}
 
   end:
-	if (!htx_add_data(htx, out))
+	if (!htx_add_data_atonce(htx, out))
 		return -1; /* Unexpected and unrecoverable error */
 	channel_add_input(chn, out.len);
 	return ret;
@@ -1805,7 +1805,7 @@
 	}
 
   end:
-	if (!htx_add_data(htx, out))
+	if (!htx_add_data_atonce(htx, out))
 		return -1; /* Unexpected and unrecoverable error */
 	channel_add_input(chn, out.len);
 	return ret;
@@ -1999,7 +1999,7 @@
 
 
   end:
-	if (!htx_add_data(htx, out))
+	if (!htx_add_data_atonce(htx, out))
 		return -1; /* Unexpected and unrecoverable error */
 	channel_add_input(chn, out.len);
 	return ret;