Modernize gem to meet feature compliance with the Datadog backend

[jedschneider]

Hi,
I've discovered that this gem is not compliant with the most recent release of the traces gem. I've pulled down the source and made some source changes and attempted to get the instrumentation to meet the supported interface outlined here: https://github.com/socketry/traces-backend-datadog/blob/c77c70d7814ee9c2fa1efbae7d462506ee5fa306/test/traces/backend/datadog.rb#L14-L30 but have run into some challenges modernizing it.

I'm not sure what the lift would be to bring this up to date and inline with the datadog backend but it would be nice to have an open telemetry protocol supported.