Record non-truncated messages length

[nicohrubec]

Should be set as `sentry.sdk_meta.gen_ai.input.messages.original_length`. In case we get a single string this will be 1.

[linear]

JS-1523

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #18970, which was included in the 10.37.0 release.