feat(outputs.event_hubs): Expose max message size batch option

[R290]

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resolves #11990

Made the existing batch option for max message size configurable using the Telegraf config. The default value of 1,000,000 is not suitable for all use cases. The basic event hub tier is 262144 bytes and the other tiers are 1048576 bytes.

[Hipska]

I don't see the default value being 1000000, so I would adapt the README and example config.

[R290]

The default setting of 1000000 is hidden in the azure-event-hubs-go library:
https://github.com/Azure/azure-event-hubs-go/blob/d88c0a040d2b0effbb71725fdc90a4793c7840dd/batch.go#L45

[Hipska]

Aha, okay. In that case I would mention that setting to 0 means using default of 1000000. As this value might change in a future version of the library.

[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_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