chore: update leader HTTP API

[shuiyisong]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Update the response format of the /admin/leader HTTP API on the meta-srv.

Before:

❯ curl localhost:3100/admin/leader
127.0.0.1:3002

After:

❯ curl localhost:3100/admin/leader | jq
{
  "leader_addr": "127.0.0.1:3002",
  "is_leader": true
} 

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the /admin/leader HTTP API within the meta-srv component by transitioning its output from a simple string to a more comprehensive JSON format. This change aims to provide richer, machine-readable information about the leader's status and address, thereby improving the API's utility and integration capabilities for clients. The modification involves defining a new data structure for the response and updating the API handler to serialize this structure into JSON.

Highlights

• API Response Format Update: The /admin/leader HTTP API on the meta-srv has been updated to return a structured JSON object instead of a plain string. The new response includes leader_addr and is_leader fields.
• New Data Structure: A new LeaderResponse struct was introduced to define the structure of the JSON response, facilitating serialization and deserialization.
• Handler Logic Refinement: The get handler for the leader API was modified to utilize the new LeaderResponse struct and axum::Json for proper JSON formatting in the HTTP response.

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[gemini-code-assist]

Code Review

This pull request updates the /admin/leader HTTP API to return a JSON object instead of plain text. This is a good improvement for machine-readability. The implementation is mostly correct, and I've suggested a small refactoring to make the code more idiomatic and concise.