Support tags/annotations per group, attribute, enum, ...

[lquerel]

There are many situations where an extensibility mechanism, such as tags, would be beneficial to support in Weaver.

Examples of tags:

• privacy_sensitivity: specifies the privacy level/sensitivity of a specific attribute.
• data_classification: specifies the classification of an attribute in a security context.
• db_mapping: specifies a specific data mapping for systems deriving a DDL from a semantic convention registry.
• proto_field_number: specifies a protobuf field number for a specific attribute.

Note: The semantics of the tags should be definable by the semantic conventions themselves. A namespace mechanism should also be used to allow the definition of tags whose semantics are defined and standardized by OTEL, while also enabling the community to define their own tags.