[feature request] merge metadata with context variables

[RogerHYang]

see e.g. #864

This means the context variables should be set at span.end() instead of at the very beginning. That way if metadata exists, it can be merged.

[dosubot]

Hi, @RogerHYang. I'm Dosu, and I'm helping the OpenInference team manage their backlog. I'm marking this issue as stale.

Issue Summary

• Feature request to allow setting context variables at span.end().
• This change would enable merging metadata with context variables if metadata is present.
• No comments or additional activity on the issue so far.

Next Steps

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