fix(processors.aws_ec2): Remove leading slash and cancel worker only if it exists

[srebhan]

Summary

Currently, tags added by the AWS EC2 processor with "metadata" contain a leading slash. This PR fixes this issue and avoids a panic when running Telegraf with --test.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #15906

[kirillmandrygin]

@srebhan I can not download build artifacts, I get 404 error

# wget https://output.circle-artifacts.com/output/job/672f852a-4cf7-44a8-8122-ae6474904c18/artifacts/0/build/dist/telegraf-1.33.0~d8ff0c4c_linux_amd64.tar.gz
--2024-10-07 08:25:10--  https://output.circle-artifacts.com/output/job/672f852a-4cf7-44a8-8122-ae6474904c18/artifacts/0/build/dist/telegraf-1.33.0~d8ff0c4c_linux_amd64.tar.gz
Resolving output.circle-artifacts.com (output.circle-artifacts.com)... 3.162.140.128, 3.162.140.108, 3.162.140.86, ...
Connecting to output.circle-artifacts.com (output.circle-artifacts.com)|3.162.140.128|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2024-10-07 08:25:10 ERROR 404: Not Found.

[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