fix: error returned to OpenTelemetry client

[jacobmarble]

The OpenTelemetry input plugin gRPC service returns the following error
to every metrics write request (this does not happen to trace or log
requests):

rpc error: code = Internal desc = grpc: error while marshaling: proto: Marshal called with nil

The solution was a single line of code, just set a pointer to an empty struct. Writing a regression test? 90 SLOC.

Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format (e.g. feat: or fix:)

[jacobmarble]

@sjwang90 please queue this bugfix for 1.20.1

[telegraf-tiger]

Looks like new artifacts were built from this PR. Get them here!

Artifact URLs

• linux_i386.tar.gzi386.rpm

• freebsd_i386.tar.gz

• i386.deb

• linux_mipsel.tar.gz

• mipsel.deb

• linux_mips.tar.gz

• mips.deb

• armv6hl.rpm

• freebsd_armv7.tar.gz

• linux_armhf.tar.gz

• armhf.deb

• armel.rpm

• linux_armel.tar.gz

• armel.deb

• ppc64le.rpm

• linux_ppc64le.tar.gz

• ppc64el.deb

• x86_64.rpm

• freebsd_amd64.tar.gz

• linux_amd64.tar.gz

• amd64.deb

• s390x.rpm

• linux_s390x.tar.gz

• s390x.deb

• static_linux_amd64.tar.gz

• aarch64.rpm

• linux_arm64.tar.gz

• arm64.deb

• windows_amd64.zip

• windows_i386.zip

• darwin_amd64.tar.gz

[srebhan]

Looks good to me.