Communicate restrictions to users

[iambriccardo]

Right now formulas can be only done between metrics of the same namespace and same metric type. In case there are any other combinations, we must tell the user this is not supported.

Further support for cross-entity/cross-namespace formulas might be coming in thee future.

[ArthurKnaus]

I think, it is important to measure how often users actually run into those cases. We should add logging for this.