fix(node): Use snake_case for Fastify's `request-handler` op.

[github-actions]

Note

The pull request "fix(node): Use snake_case for Fastify's request-handler op." was created by @onurtemizkan but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Following up: #18617

The span ops should be using snake_case as per our spec: https://develop.sentry.dev/sdk/telemetry/traces/span-operations/

[linear]

JS-1423