filecount input: support following symbolic links

[LINKIWI]

Relevant telegraf.conf:

Default config as shown in https://github.com/influxdata/telegraf/blob/master/plugins/inputs/filecount/README.md#configuration

Steps to reproduce:

Set the directory parameter to a symbolic link that links to a directory, and run Telegraf.

Expected behavior:

File counts and sizes are reported for the directory.

Actual behavior:

Dec 01 14:03:20 SOLARA telegraf[20595]: 2019-12-01T22:03:20Z E! [inputs.filecount] Error in plugin: cannot Walk non-directory: /path/to/a/symlink/that/resolves/to/a/directory

Additional info:

The filecount input depends on karrick/godirwalk, which exposes an option to follow symbolic links:

https://github.com/karrick/godirwalk/blob/master/walk.go#L35

It doesn't seem to be enabled currently:

https://github.com/influxdata/telegraf/blob/master/plugins/inputs/filecount/filecount.go#L207

I'm happy to submit a PR to address this. I'm thinking it would be best to add a new plugin config option, follow_symlinks, so to not introduce any breaking changes.

(I haven't actually tried this yet, but from a quick grok of the code, this seems like the only change needed to support symlinks.)