Add support for binop fill modifiers

[wiardvanrij]

Prometheus v3.10.0 (released 2026-02-24). The PR is #17644 (prometheus/prometheus#17644).

The proposal is prometheus/proposals#70 (https://github.com/prometheus/proposals/blob/main/proposals/0070-binop-fill-modifier.md).
It adds three modifiers: fill(), fill_left(), fill_right() for binary operations.

This requires a dep bump v3.9.1 to ≥v3.10.0 for the PromQL engine and then an update on https://github.com/thanos-io/promql-engine/ which should partially implement this from scratch to include these operations.

I'm aware that the feature is still gated as experimental on Prometheus but visa-versa it was even debatable if this had to be an experimental feature in the first place. Beyond that, I do think implementing these functions allows for a better feedback loop.

[abhinav-1504]

Hi @wiardvanrij , I've been looking into this issue and would love to work on it.
I've gone through the Prometheus PR #17644, the proposal doc (proposals#70), and the promql-engine's execution/binary/vector.go to understand the current binary matching logic.
My rough plan:

Bump Prometheus dependency from v3.9.1 to v3.10.0 in thanos main repo
Implement fill(), fill_left(), fill_right() in promql-engine's binary execution layer — specifically extending the matching logic in vector.go to inject synthetic samples when a match isn't found and a fill value is set
Propagate fill values through the logical plan nodes
Add tests covering basic fill, fill_left, fill_right, and edge cases

Before I start, a couple of questions:

Should the parser changes in promql-engine follow directly from the Prometheus v3.10.0 dep bump, or does promql-engine maintain its own parser that needs separate changes?
Is there a preferred way to handle the feature flag — mirror Prometheus's promql-binop-fill-modifiers flag or handle it differently in Thanos?

Happy to put up a draft PR once I have the dep bump + basic structure in place, to get early feedback.