fix(node-otel): Refactor OTEL span reference cleanup

[mydea]

Update: This PR was updated with new behaviour.

This PR updates the span reference cleanup to take into account if a span is/may still be referenced somewhere else.

Previously, whenever a span finished we removed the reference from the map, to clean up and avoid memory leaks.
However, it seems that sometimes spans are ended before a child span is started (at least the hooks may fire in this order). This leads to the potential case where a parent that should exist cannot be found, thus creating a new transaction instead of a span.

With this change, we keep more information in our span map, in order to clear sub-spans (=not transactions) only when the root span (=transaction) is finished.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.37 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.31 KB (0%)
@sentry/browser - Webpack (gzipped)	21.91 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	70.16 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.5 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.59 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	221.75 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	86.28 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	61.13 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.37 KB (+0.01% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	75.4 KB (0%)
@sentry/react - Webpack (gzipped)	21.94 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	93.27 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	50.88 KB (0%)

[AbhiPrasad]

Really good change, makes sense to me. We might need to do similar in the other SDKs as well.