Upgrade prometheus_exporter gem

Upgrade prometheus_exporter gem

It has memory leak fixes. They shouldn’t affect Discourse but it’s always smart to stay on top of such things.

diff --git a/prometheus_exporter_version b/prometheus_exporter_version
index f905682..1f77169 100644
--- a/prometheus_exporter_version
+++ b/prometheus_exporter_version
@@ -1 +1 @@
-0.4.7
+0.4.13

GitHub sha: 5a122897