feat(ethtool): Possibility to skip gathering metrics for downed interfaces

[zak-pawel]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

Some interfaces can expose a few thousands fields. It may not be necessary to gather and expose metrics for interfaces which are not UP.
This PR adds possibility to skip such metrics (but it defaults to exposing them - to be backward compatible):

  ## Specifies plugin behavior regarding metrics for downed interfaces.
  ## Available choices:
  ##   - expose: Telegraf will expose all available metrics for downed interfaces
  ##   - skip: Telegraf will not expose all available metrics for downed interfaces
  # interface_down_metrics = "expose"

Moreover, creation of IncludeExcludeFilter was moved from Gather to Init (input to this filter doesn't change so it makes sense to create it only once).

[powersj]

thanks - golang lint seems to be timing out here and there so I kicked it off again

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

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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