feat: opentelemetry integration

[PacoDu]

Mercurius works with existing tools like @autotelic/fastify-opentelemetry and @opentelemetry/instrumentation-graphql. But it's missing some mercurius specific spans and attributes that we can add with the opentelemetry API.

As a reference, here is an example of a mercurius trace with a gateway and a service and without any additional opentelemetry integration:

And here is an example with a simple mercurius - graphql span added to the main fastifyGraphql function and a cached attribute (see PR changes):

We need to define what we want to expose in opentelemetry traces (spans and attributes). And maybe add a telemetry option to mercurius so user can enable/disable telemetry ?

[PacoDu]

Depends on #341

[mcollina]

@Pablodu any updates on this one?

[PacoDu]

This is a demonstration of a span usage inside the mercurius library, I don't have time right now to push it further unfortunately.
If you want I can cleanup this PR and just add the tracer and the base mercurius span.

[PacoDu]

superseded by #363