Discuss limitation of GLQL transpilation to GraphQL

Currently GLQL queries are processed in the frontend and transpiled to GraphQL queries, which are then sent to the GraphQL endpoint.

This prevent scaling up GLQL as a query language as it suffers from the limitations imposed by GraphQL:

• Hard or not possible to have more complex queries: nested subqueries, parenthesis, OR operators, aggregations, math functions
• Query context limited to single project or single group
• GraphQL has inconsistent API filters and schema. Makes GLQL -> GraphQL transpilation not trivial, especially when adding new data sources
• Cannot support long async queries

All the above points are in turn limitations that we need to consider for our effort of making a GLQL-powered data explorer (Create a unified Dashboard Data Exploration and... (&17818))

The goal of this issue is to further discuss this limitations, provide some examples, so that we can later feed it into a document describing Motivation and Proposal for taking GLQL to the next level and making it into a proper API ( rather than a wrapper around GraphQL ).

Initial discussion https://docs.google.com/document/d/1eiVU3-JBZ40PPAMaESmzOTOypM6Vd3_nUJW3icClRS4/edit?usp=sharing