Add uncompressed message size stat to exporter

[plantfansam]

In Ruby, the exporter reports the size of each exported payload in bytes: https://github.com/open-telemetry/opentelemetry-ruby/blob/ad9a5c20592fe58e9a683b90195b6bb0b8766432/exporter/otlp/lib/opentelemetry/exporter/otlp/exporter.rb#L137

I think we should do something similar in our exporter. I believe we can do it here.