Add example of tracer usage

[luke88jones]

Could we add an example of creating spans around a custom process? I think it would be good to demonstrate adding information to processes which aren't covered by instrumentation modules, but the main point of interest I would have here is obtaining the tracer outside of the file it is initialised in. Should Fastify be decorated with the tracer, should it be imported or should we use getTracer() to retrieve it?

[conradthegray]

@luke88jones there is a dummy example just to check how (and if) the custom spans work, both on backend https://github.com/nearform/otlp-blueprint/blob/main/pkg-svcs/backend/lib/server.js#L14 and on frontend https://github.com/nearform/otlp-blueprint/blob/main/pkg-svcs/frontend/src/main.jsx#L17

No analysis on which approach to take to inject tracers into Fastify has been made at this point.

[luke88jones]

Thank you, I think I was concentrating more on an example away from the initial tracer initialisation as I'm interested in the injection approach the most