Expose FileSpanExporter

[drob]

The FileSpanExporter implementation here is handy, and there doesn't appear to be a standard filesystem exporter in the rest of the opentelemetry-js ecosystem.

Would you be open to any of these options?

• Contributing it to open-telemetry/opentelemetry-js, alongside ConsoleSpanExporter
• Releasing it separately, so it can be used on its own
• Exporting it, so it can be imported from @lumigo/opentelemetry

[kenfinnigan]

Hi @drob thanks for the interest in the exporter!

Though we'd be interested in contributing it to the upstream community, we don't have the bandwidth right now for such an effort.

As an interim solution, would you be willing to propose a PR exporting it so it can be imported from @lumigo/opentelemetry until its contributed? Thanks