build: `zig fetch` fails to get the SDK module

[inge4pres]

Describe the current state

Reported in Discord: a user was trying to import the library with zig fetch and faced an error.

Error building metrics examples: error.FileNotFound
thread 16476 panic: unable to find module 'opentelemetry'

This is likely due to build.zig not exporting a module, but simply an artifact for linking.

Describe the desired state

I am unable to find details in the official docs on how to make a library available to other Zig projects.

The suggestion is the Discord help is

the library's build.zig should probably be updated to export a module rather than an artifact

Not sure what this means though.

Additional context

I added the "on-hold" label because this issue should be tackled only when the project is marked as 0.1.0, therefore it's usable by other projects.

[inge4pres]

#59 addresses this.

I figured that now that we have OTLP support and Metrics are almost completed, we can try and open up the project to consumers.

I am not planning on tagging any releases just yet, simply getting very early feedback about the library.