Gather per collections stats in mongodb input plugin (#6137)

etc/telegraf.conf

@@ -3166,6 +3166,11 @@
 #
 #   ## When true, collect per database stats
 #   # gather_perdb_stats = false
+#   ## When true, collect per collection stats
+#   # gather_col_stats = false
+#   ## List of db where collections stats are collected
+#   ## If empty, all db are concerned
+#   # col_stats_dbs = ["local"]
 #
 #   ## Optional TLS Config
 #   # tls_ca = "/etc/telegraf/ca.pem"