Validate feasibility of supporting new attribute types

[pellared]

What

Investigate whether it is possible and acceptable to extend the OTel C++ API and SDK with new attribute types.

Guidelines for evolving API/SDK: https://github.com/open-telemetry/opentelemetry-specification/blob/main/oteps/4485-extending-attributes-to-support-complex-values.md#how

Prototype in OTel Go demonstrating feasibility of adding new attribute types: open-telemetry/opentelemetry-go#6809

Why

Per open-telemetry/opentelemetry-specification#4602

Follows open-telemetry/opentelemetry-specification#4614

The goal is to make sure that PRs like these are acceptable for OTel C++:

• all: drop attribute values restrictions opentelemetry-proto#707
• [EndGoal] [DoNotMerge] Extend the set of attribute value types opentelemetry-specification#4636

[lalitb]

Adding support for new types should be doable, but it would break both the API and ABI in otel-api. That means the change would have to go in with ABI v2/v3, depending on which version is current.

[marcalff]

@pellared

Support for new types should be doable, with the following caveats:

• API breaking change, can only go into ABI version 2 or above
• Representing an AnyValue in the API will add complexity
• Impact on the OTLP exporter imply adoption of a new opentelemetry-proto version (say 1.9.0)
• Impact on non OTLP exporters to define
• Adoption of new opentelemetry-proto version implies availability of an opentelemetry-collector that uses the updated protocol, for example for functional tests in opentelemetry-cpp CI.