docs: for metasrv maintenance endpoints

[tisonkun]

What's Changed in this PR

This closes #848.

Checklist

• Please ensure that the content in summary.yml matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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[MichaelScofield]

Lack of Chinese version.

[tisonkun]

@MichaelScofield Updated. The previous "Chinese version" written in English so I follow the pattern for now 🤣

cc @nicecui

[MichaelScofield]

But why is the Chinese version also written in English?

[tisonkun]

I think it's for historical reason. We should do a follow-up to translate all of admin-api to Chinese. But I tend not to block this PR.

[tisonkun]

Or I can translate the new added content anyway.

[nicecui]

that's a historical problem, pls translate the new content to Chinese, thank you very much!

[tisonkun]

@nicecui I can schedule to do it in this week, but in another PR. Just don't like to couple multiple things in one PR ..

[nicecui]

Maybe it is a bit late to discuss the APIs in this document PR...However /maintenance and /maintenance/set are not following the RESTful style of APIs...They should be:

# Get the maintenance status
GET /maintenance
# Modify the maintenance status
PUT /maintenance {status:true}

Then we can write the document as the following:

## /maintenance HTTP endpoint

The metasrv will ignore detected region failures when under maintenance. This is useful when the datanodes are planned to be unavailable for a short period of time; for example, rolling upgrade for datanodes.

### GET

The `/maintenance` endpoint accepts GET HTTP requests and you can use this endpoint to query the maintenance status of your metasrv instance.

curl -X GET http://localhost:3002/admin/maintenance

### POST
.....

[tisonkun]

@nicecui our HttpHandler abstraction doesn't pass the HttpMethod:

#[async_trait::async_trait]
pub trait HttpHandler: Send + Sync {
    async fn handle(
        &self,
        path: &str,
        params: &HashMap<String, String>,
    ) -> crate::Result<http::Response<String>>;
}

[tisonkun]

But we can change it anyway.

[tisonkun]

DB side changed. Let me update the docs today ..

[tisonkun]

Generally we should add this doc to v0.8 :D

[tisonkun]

@MichaelScofield @nicecui Updated.

[tisonkun]

@nicecui Thanks for the good catches. Updated.