fix(inputs.conntrack): resolve segfault when setting collect field

[deric]

Enabling stats feature in conntrack input by setting collect field (affected versions v1.25.0, v1.25.1) causes segfault

minimal config:

[[inputs.conntrack]]
collect = ["all"]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x1387053]

goroutine 70 [running]:
github.com/influxdata/telegraf/plugins/inputs/conntrack.(*Conntrack).Gather(0xc00046a000, {0x6cb41a0, 0xc0000c33a0})
        /go/src/github.com/influxdata/telegraf/plugins/inputs/conntrack/conntrack.go:109 +0x133
github.com/influxdata/telegraf/agent.(*Agent).testRunInputs.func2(0xc0007c2730)
        /go/src/github.com/influxdata/telegraf/agent/agent.go:419 +0x2f8
created by github.com/influxdata/telegraf/agent.(*Agent).testRunInputs
        /go/src/github.com/influxdata/telegraf/agent/agent.go:388 +0xc

fixes: #8955

The c.ps variable was not properly initialized, however during tests we're passing a mock instance, so this error was not covered by automated tests.

• Add lazy initialization when collect config is used
• collect array is checked for supported values
• Config initialization is moved out of Gather() method to Init()
• Add test case to cover the system.PS initialization
• Fix collecting both "all" and per-CPU metrics at the same time

[powersj]

Thanks for taking the time to put this up with a fix. Can you take a look at my two comments?

[deric]

@powersj Thanks for looking into this. I'm sorry, the first PR was simply bad.

[srebhan]

Very nice PR @deric! Just one small comment for the Init() code...

[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]

Looks good to me. Thanks for the quick update @deric!