[Feature Request] Structured logging (KV) support

[SpriteOvO]

Structured logging enhances traditional text-based log records with user-defined attributes. Structured logs can be analyzed using a variety of data processing techniques, without needing to find and parse attributes from unstructured text first.

With JsonFormatter (PR #69) added, the KV can be injected into the output logs as a separated JSON object for parsing by third-party programs.

Implementation

Steps / History

• Logging macros accepts a new optional parameter kv.
• Add a new kv field in struct Record.
• ... (We need to carefully consider how the API looks and is implemented)

A reference is the log kv implementation, we can see if we can improve anything base on that.