How can I add metadata while creating

the metadata attached to the nodes/documents is inserted into qdrant

No, I want to add some more information in metadata besides the default metadata that llama index extracts while creating documents and nodes

Also can you tell what is the maximum token limit of ChatMemoryBuffer?

you can attach metadata to your documents/nodes before inserting
document.metadata['key'] = 'val'

the default token limit depends on the args passed in

If no LLM is provided, the default is 3000
If an LLM is provided, the context window is used and the limit is 75% of the total context window