OpenTelemetry spec: Extended `Attribute`

[iRevive]

If I read changes correctly, Attribute is now mostly identical to AnyValue:
open-telemetry/opentelemetry-proto#707

Related:

• Prototype: Complex attributes (Option B) open-telemetry/opentelemetry-java#7661
• Prototype: Complex attributes (Option A) open-telemetry/opentelemetry-java#7632

[iRevive]

open-telemetry/opentelemetry-java#7814