MCAPWriter add_metadata

## Description

I dont know if it alreay possible, but i would like to use the "add_metadata" method of the MCAPWriter to write some metadata into the bag file, while recording. Is there a way to achieve this? 

## Related Issues
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## Completion Criteria

* [ ] Possibility to add metadata to mcap files while recording.

## Implementation Notes / Suggestions
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## Testing Notes / Suggestions
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