Support query stats

[yeya24]

The original promql engine has builtin support for query stats https://github.com/prometheus/prometheus/blob/main/util/stats/query_stats.go#L112.

It has mainly 2 parts: timings and samples.

// queryTimings with all query timers mapped to durations.
type queryTimings struct {
	EvalTotalTime        float64 `json:"evalTotalTime"`
	ResultSortTime       float64 `json:"resultSortTime"`
	QueryPreparationTime float64 `json:"queryPreparationTime"`
	InnerEvalTime        float64 `json:"innerEvalTime"`
	ExecQueueTime        float64 `json:"execQueueTime"`
	ExecTotalTime        float64 `json:"execTotalTime"`
}

type querySamples struct {
	TotalQueryableSamplesPerStep []stepStat `json:"totalQueryableSamplesPerStep,omitempty"`
	TotalQueryableSamples        int64      `json:"totalQueryableSamples"`
	PeakSamples                  int        `json:"peakSamples"`
}

// BuiltinStats holds the statistics that Prometheus's core gathers.
type BuiltinStats struct {
	Timings queryTimings  `json:"timings,omitempty"`
	Samples *querySamples `json:"samples,omitempty"`
}

It would be nice to support capturing those stats in this query engine.

[yeya24]

Since this query engine is different from the old engine in terms of the query execution stages, I feel we don't have to support query timings for now. But it would be nice to support query samples first.

[fpetkovski]

Can we solve this through #106 ?

[yeya24]

I think #106 has a larger scope than this one.
This issue is mainly to support Prometheus query stats in the engine because currently if I enable query stats the engine panics.

[fpetkovski]

Got it, makes sense.

[sahnib]

I would like to contribute to adding support for query stats in the engine.