fix(js): Fix span metrics examples

[mydea]

This fixes some of the code examples around span metrics in JS.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 7, 2025 7:01am

2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
changelog	⬜️ Ignored (Inspect)	Visit Preview		Apr 7, 2025 7:01am
develop-docs	⬜️ Ignored (Inspect)	Visit Preview		Apr 7, 2025 7:01am

[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	11.13MB	-9 bytes (-0.0%) ⬇️
sentry-docs-client-array-push	9.47MB	-6 bytes (-0.0%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: sentry-docs-server-cjs

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
1729.js	-3 bytes	1.75MB	-0.0%
../instrumentation.js	-3 bytes	1.08MB	-0.0%
9523.js	-3 bytes	1.05MB	-0.0%

view changes for bundle: sentry-docs-client-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
static/chunks/pages/_app-*.js	-3 bytes	868.7kB	-0.0%
static/chunks/8165-*.js	-3 bytes	410.1kB	-0.0%
server/middleware-*.js	-5.55kB	1.0kB	-84.74%
server/middleware-*.js	5.55kB	6.55kB	555.3% ⚠️
static/wu4DsJVlRdbX5IOwSDI6R/_buildManifest.js (New)	578 bytes	578 bytes	100.0% 🚀
static/wu4DsJVlRdbX5IOwSDI6R/_ssgManifest.js (New)	77 bytes	77 bytes	100.0% 🚀
static/tOeYYeBpLdzU-*.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️
static/tOeYYeBpLdzU-*.js (Deleted)	-578 bytes	0 bytes	-100.0% 🗑️

[codyde]

Appreciate the help on this, LGTM!