fix: Clear error message when provided config is not a text file

[sspaink]

resolve: #8753

The issue was providing a non text file to Telegraf as a config would cause the error Error parsing data: line 1: invalid TOML syntax which isn't that helpful. I couldn't find a good way to detect the type of a file without reading it, you could detect it by extension: https://pkg.go.dev/mime?utm_source=godoc#TypeByExtension but testing this on windows I wouldn't get a result for .toml. So I chose to use: https://pkg.go.dev/golang.org/x/tools/godoc/util?utm_source=godoc#IsText which will look at the contents of the file to determine if it is a text file.

This does mean that depending on the binary size it could be slow to print this error message, but hopefully the error message will help regardless. Because when providing a URL the request header is updated with req.Header.Add("Accept", "application/toml"), I thought this might suffice and the extra check if it is a text file shouldn't be necessary and its the servers responsibility. But open to consider this differently.

[Hipska]

when providing a URL the request header is updated with req.Header.Add("Accept", "application/toml"), I thought this might suffice and the extra check if it is a text file shouldn't be necessary and its the servers responsibility.

I agree with this, they will still receive the message that it isn't valid TOML.

[sspaink]

Weird, the tests in Windows are failing in the CI because of this change but I can't reproduce it locally on a Windows machine (https://app.circleci.com/pipelines/github/influxdata/telegraf/12585/workflows/9895e703-3964-4ddf-ae2a-91aef8ecaab1/jobs/203622). I thought maybe it had to do with the encoding because util.IsTest requires utf8 but before failing it prints the mimetype correctly as text/plain; charset=utf-8.

Updated the logic to use the mimetype returned by http.DetectContentType as a potential alternative.

[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.

👍 This pull request doesn't change the Telegraf binary size

📦 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_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