Revisit enum attribute value stability

[lmolkova]

Per current spec enums can change in any possible way:

opentelemetry-specification/specification/versioning-and-stability.md

Lines 216 to 224 in e69c7a0

	Things not listed in the above are not expected to remain stable via semantic
	convention and are allowed (or expected) to change. A few examples:
	- The values of attributes
	- Specifically for `enums` the list of allowed values is expected to change
	overtime.
	- Even for `enums` that limit allowed values to semconv, some may need to
	updated values in the future. Tooling should expect unknown values.
	- The links attached to a span

this creates a problem for instrumentation libraries and semconv artifacts: if existing value is renamed, it causes breaking changes on the API surface or binary-incompatibility and runtime breaking changes.

Proposal:

• guarantee stability of existing enum values
• allow list to grow

Related: open-telemetry/semantic-conventions#670