Add latency unit field in JSON logs

[Babai-69]

Added latencyUnit field in JSON logs to make latency values explicit.

Changes made:

• Added ResponseUnit field in log struct
• Added "µs" in gRPC logs
• Added "µs" in HTTP middleware logs
• Updated related test files

[Umang01-hash]

Hey @Babai-69 !
Thanks for the effort to make GoFr better. Your PR currently have this issue that the PKG Unit Testing step in workflow is failing with the following error :

Also it will be nice if you can attach the screenshot for the new grpc and http logs so that other reviewers can easily verify the changes.

[Babai-69]

Thanks for pointing it out. I added the missing ResponseUnit field in RequestLog and pushed the fix.

Attaching sample screenshots for updated HTTP and gRPC logs showing the new latencyUnit field for easier verification.