Migrate to @fastify/otel instrumentation

[andreiborza]

Description

Fastify has taken ownership over the fastify Otel instrumentation and migrated the integration out of the opentelemetry-js-contrib repo into their own repo.

Once open-telemetry/opentelemetry-js-contrib#2652 lands, the current instrumentation, which we are using, will be disabled by default and removed completely by March 25, 2025.

Solution Brainstorm

Migrating to @fastify/otel is inevitable, but if integration proves difficult we have to specifically enable the integration,

[mydea]

is there something that speaks against just switching out the instrumentation right now?

[mydea]

hmm, this only supports fastify 5+ 😬

[mydea]

See: fastify/otel#10

[mydea]

After talking about this some more, our idea is to:

1. Switch to use @fastify/otel instead of @opentelemetry/instrumentation-fastify
2. Vendor the current state of @opentelemetry/instrumentation-fastify into the node package (note: We'll need to also inline some of the types accordingly). Limiting the range this applies to to >=3<=4.

We can use this blueprint for other similar situations too, in the future! This way, we continue to support fastify 3+, but can stay on the latest version of the official instrumentation and get improvements from there.