Fill operator support in PromQL

Dash0 has implemented support for the fill modifier in PromQL binary operations.

The problem being addressed: when performing binary operations in PromQL, results silently disappear when one side lacks matching time series. For example, dividing requests by capacity fails to display instances that haven't yet reported capacity data — creating gaps in dashboards and blind spots in alerting systems.

The solution introduces the fill modifier with three variants:

• fill(<value>) — applies a default value to whichever side is missing
• fill_left(<value>) — substitutes only when the left operand has no match
• fill_right(<value>) — substitutes only when the right operand has no match

Important caveat: When both sides of an operator lack data, no time series is generated.

The feature is now available through Dash0's UI and API. The implementation resulted from collaboration with the Prometheus community.