Onboard with OSD multiple data source feature

[zhongnansu]

When OSD MD feature is enabled. All visualizations in native OSD will support its use case. User experience side, there shouldn't be a difference for them. Since gantchart is also a visualization. Could you evaluate the changes of user experience?

For integrating with MD feature. We have provided API to be able to get a data source client instead of the default legacy client in gantchart. Logically, all places that interacts with OS will need to switch to use datasource client on demand. Since I am not familiar with the code base, I only found 1 places that does the actual search. Am I missing anything?

https://github.com/opensearch-project/dashboards-visualizations/blob/main/gantt-chart/server/routes/index.ts#L42

[joshuali925]

Gantt chart uses IAggConfigs to get fields and context.core.opensearch.legacy.client.callAsCurrentUser() to send search requests

If user selects an index pattern from a different data source, I'm assuming IAggConfigs would have its metadata? It's used here

dashboards-visualizations/gantt-chart/public/components/gantt_chart_editor.tsx

Line 19 in 029f174

const fieldOptions = aggs.indexPattern.fields.map((field: Field) => {