feat(RFC): Add Internal Trace Model.

[hdost]

Provides a path forward for integrating sources and sinks for Tracing.

Relates #1444, #17307, #17308

[bruceg]

We discussed this internally and agree with the general direction of this proposal. It makes sense to adopt the the OpenTelemetry data types as the internal model, since the OTel project has defined many mappings to and from their data model for other trace types. So it stands as a useful intermediate representation for many kinds of traces.

However, if we are going to adopt that data model, we would prefer it be implemented in a more structured form. Specifically, we would want all of the data structures, were possible, to become fixed Rust structs instead of a generic map type. This would echo our metric types, which can represent a variety of different kinds of metrics with a fixed structure. This will simplify all kinds of code managing conversion to and from other data types, such as protobufs or other trace models, since they can rely on the presence of certain fields at a source code level.

As such, we would like to see the above reflected in the RFC. It should contain source code specifics of what the structures would look like and, if appropriate, what methods would be made available for manipulating them. Since this isn't a completely different direction, we don't think it requires a complete overhaul of this document but it does require changing the core data types, which means that consideration is very much in scope.

Some examples from existing RFCs to show what that might look like:

• https://github.com/vectordotdev/vector/blob/master/rfcs/2022-10-12-14742-flexible-metric-tags.md
• https://github.com/vectordotdev/vector/blob/master/rfcs/2022-07-28-13691-registered-internal-events.md
• https://github.com/vectordotdev/vector/blob/master/rfcs/2022-02-23-9481-config-schema.md

Many other RFCs contain Rust code samples, as we intend for these documents to be very opinionated as to the target solution, with many of the implications worked out.

[hdost]

Thank you for the thorough review, I'll take some time to reformulate with all that in mind.