Support for on demand / queryable data sources

[andrewringler]

It looks like in all of the examples the data sources are fully read into the client (in-memory) and are needed to create the React Chart Editor.

I have a large database I want to query and I cannot load the entire database into memory on the client. Is there a way to specify the table/column name metadata only in data sources so that they appear correctly in the Traces UI, but not actually load the data or query the database until we have selected the traces?

Ideally I could specify the metadata in data sources (which would be used by the Traces UI), and also specify getters for the data, which would only be used by the chart section (to get the data it needs, depending on trace and filter definitions).