Implement thanos promql engine fallback logic #6624
Description
Is your feature request related to a problem? Please describe.
Thanos promql engine used to always fallback to the Prometheus engine if certain expressions are not supported.
However, latest version of Thanos promql engine removes this fallback logic in thanos-io/promql-engine#518 and now it is the application's responsibility to decide whether to fallback or not.
To unblock upgrading thanos promql engine in Cortex, we need to implement the fallback logic in Cortex.
Describe the solution you'd like
Thanos moves the fallback logic in Thanos itself as well. thanos-io/thanos#8118 is an example PR of how to do the fallback.