fix: SNMP MIBs: use the correct path when evaluating symlink

[reimda]

related #10674

This PR changes handling of symlinks to files when loading MIBs. Previously it would try to evaluate the parent directory's path as a symlink instead of the symlink's path.

This change should remove many "Couldn't stat target" warnings in logs like the following:

2022-02-17T18:06:33Z W! [inputs.snmp] Couldn't stat target /usr/share/snmp/mibs

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
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Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[ericjohncarlson]

@reimda I can confirm that this version works properly and does not throw the Couldn't stat target error that was called out in the original #10674 issue. Thanks!

[MyaLongmire]

Thanks for figuring this out!

[reimda]

@ericjohncarlson Thanks for testing so quickly. Did you also have the "translating: cannot make X numeric" error and did this PR fix that too?

[ericjohncarlson]

@ericjohncarlson Thanks for testing so quickly. Did you also have the "translating: cannot make X numeric" error and did this PR fix that too?

I'm sorry I did not have that translating error, mine was purely associated to the path issue and not being able to find the .mib files.

[srebhan]

Thanks for the patch @reimda! One question, can we add a unit-test for this case in order to make sure we don't break it in the future?

[Hipska]

According to all the responses in #10674, it seems like this PR fixes the "cannot stat" issue. Well done!

[srebhan]

I don't want to hold this up due to a superfluous newline. ;-)
LGTM! Thanks @reimda for fixing!