fix(metrics): Update return type of beforeSendMetric

[chargome]

missed this in #18088

[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	8,806	-	9,040	-3%
GET With Sentry	1,391	16%	1,334	+4%
GET With Sentry (error only)	6,092	69%	5,951	+2%
POST Baseline	1,212	-	1,193	+2%
POST With Sentry	527	43%	501	+5%
POST With Sentry (error only)	1,063	88%	1,044	+2%
MYSQL Baseline	3,349	-	3,259	+3%
MYSQL With Sentry	513	15%	450	+14%
MYSQL With Sentry (error only)	2,746	82%	2,668	+3%

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