fix(serializers.prometheus): Use legacy validation for metric name

[srebhan]

Summary

With PR #16453 we bumped the prometheus package which in turn changed the validation scheme for metric-names and labels to only being UTF-8 conform. However, while changing the label validation scheme back to "legacy" the metric name validation was not. This PR fixes the metric name validation and reverts the check to "legacy". This PR is also relevant for the prometheusremotewrite serializer as it reuses the sanitization code.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #16550

[telegraf-tiger]

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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