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

[onurtemizkan]

Following up: #18617

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

Closes #18730 (added automatically)

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	9,411	-	9,038	+4%
GET With Sentry	1,656	18%	1,667	-1%
GET With Sentry (error only)	5,978	64%	6,029	-1%
POST Baseline	1,181	-	1,193	-1%
POST With Sentry	585	50%	586	-0%
POST With Sentry (error only)	1,042	88%	1,038	+0%
MYSQL Baseline	3,331	-	3,377	-1%
MYSQL With Sentry	467	14%	497	-6%
MYSQL With Sentry (error only)	2,712	81%	2,733	-1%

View base workflow run

[Lms24]

thanks!