// ... in your OTEL setup
const FastifyOtelInstrumentation = require('@fastify/otel');

// If serverName is not provided, it will fallback to OTEL_SERVICE_NAME
// as per https://opentelemetry.io/docs/languages/sdk-configuration/general/.
const fastifyOtelInstrumentation = new FastifyOtelInstrumentation({ servername: '<yourCustomApplicationName>' }); 
fastifyOtelInstrumentation.setTracerProvider(provider)

module.exports = { fastifyOtelInstrumentation }

// ... in your Fastify definition
const { fastifyOtelInstrumentation } = require('./otel.js');
const Fastify = require('fastify');

const app = fastify();
// It is necessary to await for its register as it requires to be able
// to intercept all route definitions
await app.register(fastifyOtelInstrumentation.plugin());

// automatically all your routes will be instrumented
app.get('/', () => 'hello world')
// as well as your instance level hooks.
app.addHook('onError', () => /* do something */)

// you can also scope your instrumentation to only be enabled on a sub context
// of your application
app.register((instance, opts, done) => {
    instance.register(fastifyOtelInstrumentation.plugin());
    // If only enabled in your encapsulated context
    // the parent context won't be instrumented
    app.get('/', () => 'hello world')

}, { prefix: '/nested' })