Add changelog entry for prometheus#1439

Signed-off-by: Ben Kochie <superq@gmail.com>

CHANGELOG.md

@@ -15,6 +15,7 @@
 * [CHANGE] Refactor mdadm collector #1403
 * [FEATURE] Add new schedstat collector #1389
 * [ENHANCEMENT] Include additional XFS runtime statistics. #1423
+* [ENHANCEMENT] Report non-fatal collection errors in the exporter metric. #1439
 * [BUGFIX] Renamed label `state` to `name` on `node_systemd_service_restart_total`. #1393
 * [BUGFIX] Fix netdev nil reference on Darwin #1414
 * [BUGFIX] Strip path.rootfs from mountpoint labels #1421