fix(inputs.mongodb): Do not dereference nil pointer if gathering database stats fails

[dkhamitov]

Summary

Occasionally happens on our clusters, like every 2-4 weeks. The error message has never been seen as it gets logged at Debug level, which has also been changed to Error in this commit. Please let me know if the log level update is not an issue as I can see some errors are only logged at Debug level.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x29ed457]

goroutine 4494435 [running]:
github.com/influxdata/telegraf/plugins/inputs/mongodb.(*Server).gatherData (0xc000f3c900, {0x6d39940, 0xc000667a80}, 0x0, 0x1, 0x1, 0x1, {0x9ec1ed0, 0x0, 0x0})
        /go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb_server.go:338 +0x5f7
github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather.func1(0xc000813480?)
        /go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:180 +0x1a9
created by github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather
        /go/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:171 +0x65

Checklist

• No AI generated code was used in this PR

Related issues

Resolves #16329

[srebhan]

@dkhamitov could you please open an issue additional to this PR describing the issue? This will ease finding this solution for other users...

[srebhan]

Thank you for your fix @dkhamitov! I do have one small comment in the code...

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

📦 Click here to get additional PR build artifacts

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 @dkhamitov!