doc(opentelemetry): add tracing documentation with zipkin and opentelemetry

[PacoDu]

Waiting for @opentelemetry/instrumentation-graphql release.

[PabloSzx]

Maybe it's better for open-telemetry.md to be in docs/integrations/

[PacoDu]

TODO: switch to @autotelic/fastify-opentelemetry

[mcollina]

@adamovittorio PTAL

[adamovittorio]

@projectjudge FYI, we can try it with the gcp/prometheus exporter and see if we can remove the custom code for the headers propagation in fastify. Then in the ctx we have access to the tracer that we can use to instantiate custom spans.

[jonnydgreen]

@adamovittorio for sure, we can give it a go and update the decorator for our custom spans as well if needed

[PacoDu]

I switched to @autotelic/fastify-opentelemetry for the documentation.

It's not perfect, here is the trace I get with the configuration described in the documentation:

For the _service { sdl } requested by the gateway from service-add I've a clean trace:

But for the example query { add(x: 1, y: 2):

[PacoDu]

Ok ... the gateway doesn't work as expected with a query returning a Scalar, it returns null. Modifying the schema to return an object fixed the issue.
Edit: the bug is related to mercurius-js/mercurius-gateway#23 service-add.js needs to extend the Query otherwise it doesn't work.

Opentelemetry is perfectly working 🎉 :

[mcollina]

lgtm

[mcollina]

I think we should document that this would not work with graphql-jit. Or does it?

[PacoDu]

I think we should document that this would not work with graphql-jit. Or does it?

I didn't test this with jit enabled, I suppose it will not work for the graphql trace, but we will still get the http trace at least.
Maybe we can add some mercurius specific spans / attributes in another PR.

[PacoDu]

Normal:

Jitted:

Edit: I didn't enable the custom span compute-add in the screenshots above, but it works with the jitted version:

[mcollina]

lgtm