Adopt GDI specification v1.6.0

[pellared]

Adopt https://github.com/signalfx/gdi-specification/releases/tag/v1.6.0

Blocked by:

• Audit Compliance GDI specification pre-v1.7.0: Behaviors #554
• Audit Compliance GDI specification pre-v1.7.0: Configuration #555
• Audit Compliance GDI specification pre-v1.7.0: Repository #556
• Audit Compliance GDI specification pre-v1.7.0: Semantic Conventions #557
• Audit Compliance GDI specification pre-v1.7.0: Versioning #558