fix(inputs.netflow): Fix sFlow metric timestamp

[Hipska]

Required for all PRs

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

Currently, the sysUptime field was used, but that isn't a unix timestamp:

The time (in hundredths of a second) since the network management portion of the system was last re-initialized.

This PR rectifies this and uses the same as for the netflow decoders.

-netflow,source=127.0.0.1,version=sFlowV5 sys_uptime=12414u,agent_ip="192.168.119.184",agent_subid=100000u,seq_number=2u,in_snmp=3u,port_name="eno1",ip_version="IPv4" 12414000000
+netflow,source=127.0.0.1,version=sFlowV5 sys_uptime=12414u,agent_ip="192.168.119.184",agent_subid=100000u,seq_number=2u,in_snmp=3u,port_name="eno1",ip_version="IPv4" 16983296530

[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_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
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

[srebhan]

Thanks for spotting and fixing this @Hipska!