feat: store every usage information in conversation

[tusharmath]

Conversation currently only stores the last usage information. This makes it difficult to assess cache hit/miss issues in dumps. Ideally for each request we make we should store the usage information so that we can see - token usage and cost information in detail.

Technical Requirements

• Store it in the meta field
• Add a timestamp field in the data.
• Keep it open for extensions (we might want to add more fields)

Related #1485

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