Adopt GDI specification v1.6.0

Adopt https://github.com/signalfx/gdi-specification/releases/tag/v1.6.0

Blocked by:
- https://github.com/signalfx/splunk-otel-js/issues/977
- https://github.com/signalfx/splunk-otel-js/issues/978
- https://github.com/signalfx/splunk-otel-js/issues/979
- https://github.com/signalfx/splunk-otel-js/issues/980
- https://github.com/signalfx/splunk-otel-js/issues/981