add undocumented fd-usage metric

description is copied from dnsdist
https://dnsdist.org/statistics.html#fd-usage

diff --git a/pdns/recursordist/docs/metrics.rst b/pdns/recursordist/docs/metrics.rst
index ba40f59a9a0899353cefeb480e08f77bc424c576..f8993129cf3204f554e903f8efa4c627826c8e64 100644 (file)
--- a/pdns/recursordist/docs/metrics.rst
+++ b/pdns/recursordist/docs/metrics.rst
@@ -487,6 +487,10 @@ failed-host-entries
 ^^^^^^^^^^^^^^^^^^^
 number of servers that failed to resolve
 
+fd-usage
+^^^^^^^^
+Number of currently used file descriptors.
+
 ignored-packets
 ^^^^^^^^^^^^^^^
 counts the number of non-query packets received   on server sockets that should only get query packets