feat(inputs.modbus): add pause after connect config parameter

[joerg-git]

Required for all PRs

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

resolves feature request #11919

Allows to add a pause (warm-up time) for the slave device after modbus connect. E.g. Huawei solar inverter ethernet dongles need this pause to response the first request.

[joerg-git]

Two checks failed:

1. test-go-windows: FAIL: plugins/inputs/powerdns
2. make-test-integration: FAIL: TestConnectAndWriteIntegrationX509Auth

My fault?

Edit: all checks passed now.

[telegraf-tiger]

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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

[powersj]

Thanks for taking the time to put this up.

I am concerned that this will lead to a lot of "interval took too long" messages if set to high as you mention in the readme.

[joerg-git]

I am sure that users who need this feature have thought about the duration of Modbus requests beforehand. They will be able to handle it. Thank you for accepting my request.