Host operating system: output of uname -a

Linux debian 4.9.0-6-amd64 #1 SMP Debian 4.9.88-1+deb9u1 (2018-05-07) x86_64 GNU/Linux

node_exporter version: output of node_exporter --version

node_exporter, version 0.16.0 (branch: master, revision: ac5a981)
build user: root@debian
build date: 20180706-16:39:05
go version: go1.10.3

node_exporter command line flags

--collector.mountstats --log.level=debug

Are you running node_exporter in Docker?

No

What did you do that produced an error?

I mounted the same device twice, both mounts were on the same mountpoint. The first mount was done using nfs version 3 and the other was done using nfs version 4.
Command: mount -t nfs -o vers=3 127.0.0.1:/var/nfs /mnt/nfs
Command: mount -t nfs -o vers=4 127.0.0.1:/var/nfs /mnt/nfs

What did you expect to see?

The mounts have some differences(e.g age, port, events stats). Considering that these metrics are different I expected to see a separate line for each nfs mount in node_exporter metrics.

What did you see instead?

I saw metrics for only one mount.
node_mountstats_nfs_age_seconds_total{export="127.0.0.1:/var/nfs"} 499
# HELP node_mountstats_nfs_event_jukebox_delay_total Number of times the NFS server indicated EJUKEBOX; retrieving data from offline storage.
# HELP node_mountstats_nfs_event_vfs_read_page_total Number of pages read directly via mmap()'d files.
# TYPE node_mountstats_nfs_event_vfs_read_page_total counter
node_mountstats_nfs_event_vfs_read_page_total{export="127.0.0.1:/var/nfs"} 0
# HELP node_mountstats_nfs_event_vfs_read_pages_total Number of times a group of pages have been read.
# TYPE node_mountstats_nfs_event_vfs_read_pages_total counter
node_mountstats_nfs_event_vfs_read_pages_total{export="127.0.0.1:/var/nfs"} 0
# HELP node_mountstats_nfs_event_vfs_update_page_total Number of updates (and potential writes) to pages.
# TYPE node_mountstats_nfs_event_vfs_update_page_total counter
node_mountstats_nfs_event_vfs_update_page_total{export="127.0.0.1:/var/nfs"} 0

My current approach to solving this problem is to have the mounstats_linux.go compare the device name, mountpoint and port. If all those values are the same then the collector will skip it as a duplicate, if not then it will show the metrics. Do you think this is a good solution? I can make a pull request if the approach seems fine.