[Do not merge] Agent api prototype

[LikeTheSalad]

Prototype that should cover what's being discussed here.

• A Kotlin factory method that returns OpenTelemetryRumBuilder, which to me it seems to be the simplest way of using the agent as there's only one method that users would need to be aware of. It also contains config params for the default instrumentations.
• Following the diagram, it removes some API surface from OpenTelemetryRumBuilder.
• Following the diagram, It moves the "unserializable" config params from OtelRumConfig into OpenTelemetryRumBuilder.
• Following the diagram (kind of, as the interface initially named SessionManager is now changed to SessionProvider Session handling implementation opentelemetry-js-contrib#2358) it sets the SessionProvider as the only session-related type in core, and moves all of our session-related types to the agent so that the existing session handling tools (the ones that create sessions based on a timeout) are provided by the agent, and "power-users" can provide their own session handling logic directly to core, if needed.
• Following the diagram, it creates EndpointConfig and a default impl that provides HTTP endpoints.
• Adds the OTLP exporters dependency to the agent module and makes the agent to automatically set otlp-http span and log exporters that use the provided endpoint config params.
  Here we can see how this agent API is used in our demo app.

[breedx-splk]

I like the idea of having a simple and direct configuration approach, but we should still make it easy for users to configure separate endpoints for logs and spans. We can't assume that they're the same.

[LikeTheSalad]

Got it, yeah it makes sense. Right now the EndpointConfig interface provides a separate getter for each, although it's not the most user-friendly approach having to implement it just to provide different URLs. Do you have any preferences of what could be a nice way to address this? The one thing I was thinking right now was to remove the EndpointConfig param and instead add string params for each url, and maybe a "defaultUrl" one that gets overridden if there's a value provided for a specific signal url.

[bidetofevil]

Separate for each and fallback to the default sounds good. There aren't so many signals that it would be onerous to specify the URL twice on init if they have a custom one

[breedx-splk]

as a user, when I see that I can pass in a "rum config" object, I instantly wonder what's in there. ...but then I see this other stuff alongside it and wonder even more why some of this configurable stuff is in the config but some of it is passed directly to the builder. Seems inconsistent.

[LikeTheSalad]

That's true, although I'd argue that we're already in a similar situation by having an OtelRumConfig object passed to the OpenTelemetryRumBuilder which itself has some config options too! 😅

At some point, I was thinking about OtelRumConfig as a place for flags so that we could switch things on/off and also provide a serializable part of the config which might be useful for debug logs, and then the rest of config options in the builder as a place for more complex objects... Although maybe we would also like to show something else in the debug logs apart from just flags, in which case I'm not sure how can I justify having both tbh. What's your view on this?

[bidetofevil]

Is this more of a concern about what config goes into what object? It feels OK to have both if we can clearly separate which configs go into which object. But yeah, if OtelRumConfig is public, I would assume that folks can pass in an instance configured however they want and assume that it'll work.

[bidetofevil]

Added some suggestions but I think this is an incremental improvement that we can build on.

Between this PR and this, a larger discussion can probably be had about separating the project into layers so that they can be individually configured and used. There's still a bit too much mixing of concerns between the Agent, the platform services it provides, the SDK instance it wraps, and the instrumentation that depend directly on one or more of these layers. Until the separation is more formal, it'll be hard to get away from some of these challenges of partial configuration and reuse.

[bidetofevil]

checkNotNull() is preferred if you have to verify something is non-null

An alternative is to allow these to be null and handle it downstream if we can envision a case where the agent can work without some of these things

[bidetofevil]

Is this more of a concern about what config goes into what object? It feels OK to have both if we can clearly separate which configs go into which object. But yeah, if OtelRumConfig is public, I would assume that folks can pass in an instance configured however they want and assume that it'll work.

[bidetofevil]

Separate for each and fallback to the default sounds good. There aren't so many signals that it would be onerous to specify the URL twice on init if they have a custom one

[bidetofevil]

This is effectively a static singleton. Are we OK with this (especially for testing), or do we want to wrap all the state up in its own object so we can replace it at runtime?

[LikeTheSalad]

I think we got a rough idea from this draft, so I'll close it and start working on small changes towards some form of initialization API on other PRs.