Return metadata on each call to the engine
This MR updates the return type of the engine classes to return the following metadata:
- length of the prefix/suffix in characters and tokens
- total lenght of parsed imports in characters and tokens
- total lenght of imports added to the prompt in characters and tokens
- model/engine name
- language
We can add additional metadata and log it with any follow up MR.
Blocked by !243 (merged)
Closes #193 (closed)