Feature: MDS client enhancement

[SuZhou-Joe]

Description

[Describe what this change achieves]

Issues Resolved

[List any issues this PR will resolve]

Check List

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[codecov-commenter]

Codecov Report

Merging #706 (6386623) into 2.x (4979092) will decrease coverage by 0.98%.
The diff coverage is n/a.

❗ Current head 6386623 differs from pull request most recent head 165b91c. Consider uploading reports for the commit 165b91c to get more accurate results

@@            Coverage Diff             @@
##              2.x     #706      +/-   ##
==========================================
- Coverage   63.37%   62.40%   -0.98%     
==========================================
  Files         341      337       -4     
  Lines       11545    11435     -110     
  Branches     2108     2211     +103     
==========================================
- Hits         7317     7136     -181     
- Misses       3654     3737      +83     
+ Partials      574      562      -12     

see 21 files with indirect coverage changes

[zengyan-amazon]

data source plugin registered client objects at https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/data_source/server/plugin.ts#L99 , not sure if that can be reused

[SuZhou-Joe]

Good point and plugin should add dataSource plugin as requiredPlugins, add this change into this PR.

[ruanyl]

What do you think of using Proxy instead of overwriting the original method? Something like this:

    const proxy = new Proxy(legacyClient, {
      get(target, prop) {
        if (prop === 'asScoped') {
          return (request) => {
            if (/* data source id presents */) {
              return {
                callAsCurrentUser: async () => {/.../},
                callAsInternalUser: async () => {/.../},
              };
            }
            return target.asScoped(request);
          };
        }
        return Reflect.get(...arguments);
      },
    });
    // then use legacyClient proxy
    const indexService = new IndexService(proxy);

[ruanyl]

I remember you mentioned the concern of putting _data_source_id_ in http headers and the challenge of moving it to url search parameters.

Since we modified the original request here anyway(though it looks a bit hacky as registerRouteHandlerContext is not intent for this). If we end up with modifying the request object, perhaps we can also do this to handle data source id in search parameters:

    core.http.registerRouteHandlerContext(`${props.pluginId}_MDS_CTX_SUPPORT`, (ctx, req) => {
      req[`${props.pluginId}_context`] = ctx;
      if (req.url.searchParams.has('__data_source_id__')) {
        // add data source id to request object so later access
        req[`${props.pluginId}_data_source_id`] = req.url.searchParams.get('__data_source_id__');
        // then we can remove it from search params so it won't bother router validators.
        req.url.searchParams.delete('__data_source_id__');
      }
      return {};
    });