feat(inputs.nginx_plus_api): Gather limit_reqs metrics

[glennlod]

Required for all PRs:

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

This was an issue we encountered ourselves thus we didn't create an issue but fixed it immediately. The limit reqs information is very important for NGINX as we need to be able to monitor the amount of requests that were dropped due to those limits.

Description:
Added a new API call to this plugin to gather the limit_reqs metrics that NGINX already exposes.

[telegraf-tiger]

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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

[srebhan]

Looks good to me. Thanks for the quick response @glennlod!

[srebhan]

!retry-failed