feat(inputs.statsd): Expose allowed_pending_messages as internal stat

[jdheyburn]

Summary

allowed_pending_messages is a config described as:

  ## Number of UDP messages allowed to queue up, once filled,
  ## the statsd server will start dropping packets

Currently statsd_pending_messages is exported which allows you to see the number of pending messages. However exporting the config value for allowed_pending_messages is helpful to then set up alerting on when that limit has been reached.

I'm open to changing the name of the stat away from max_pending_messages. I don't mind setting it to allowed_pending_messages but then I think I would need code similar to below to avoid the conflict in names. MaxPendingMessages to me indicates that pending messages cannot go above this value.

	s.AllowedPendingMessagesStat = selfstat.Register("statsd", "allowed_pending_messages", tags)
	s.AllowedPendingMessagesStat.Set(int64(s.AllowedPendingMessages))

Checklist

• No AI generated code was used in this PR

Related issues

resolves #15686

[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

[srebhan]

Thanks @jdheyburn!