Migrate ddtrace(v1) to datadog(v2).

[yusuke-ota]

Why

Migrate ddtrace(datadog v1) gem to datadog v2 gem.

📝 ddtrace gem was renamed to datadog when ddtrace version bump to 2.0.0.

What

• Replace the ddtrace gem with the datadog gem.
• Check that the ddtrace constant variables used in the code have not been deleted in the datadog gem.
• Bump up supported Ruby and Rake versions because they are too old to run the tests.

[gedorinku]

datadog 2.x includes many breaking changes. It is necessary to address all of them.
https://github.com/DataDog/dd-trace-rb/blob/release/docs/UpgradeGuide2.md

For example, this code uses Datadog::Tracing::Span#span_id, but it has been renamed in datadog 2.x.

opencensus-datadog/lib/opencensus/trace/exporters/datadog/converter.rb

Line 45 in f14dcae

span_id: span.span_id.to_i(16),