Alerting: rule backtesting API (#57318)

* Implement backtesting engine that can process regular rule specification (with queries to datasource) as well as special kind of rules that have data frame instead of query.
* declare a new API endpoint and model
* add feature toggle `alertingBacktesting`

pkg/services/ngalert/backtesting/eval_data.go

@@ -0,0 +1,96 @@
+package backtesting
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"github.com/grafana/grafana-plugin-sdk-go/data"
+
+	"github.com/grafana/grafana/pkg/expr"
+	"github.com/grafana/grafana/pkg/expr/mathexp"
+	"github.com/grafana/grafana/pkg/services/ngalert/eval"
+)
+
+// DataEvaluator is evaluator that evaluates data
+type dataEvaluator struct {
+	refID              string
+	data               []mathexp.Series
+	downsampleFunction string
+	upsampleFunction   string
+}
+
+func newDataEvaluator(refID string, frame *data.Frame) (*dataEvaluator, error) {
+	series, err := expr.WideToMany(frame)
+	if err != nil {
+		return nil, err
+	}
+	for _, s := range series {
+		s.SortByTime(false)
+	}
+
+	return &dataEvaluator{
+		refID:              refID,
+		data:               series,
+		downsampleFunction: "last",
+		upsampleFunction:   "pad",
+	}, nil
+}
+
+func (d *dataEvaluator) Eval(_ context.Context, from, to time.Time, interval time.Duration, callback callbackFunc) error {
+	var resampled = make([]mathexp.Series, 0, len(d.data))
+
+	iterations := 0
+	for _, s := range d.data {
+		// making sure the input data frame is aligned with the interval
+		r, err := s.Resample(d.refID, interval, d.downsampleFunction, d.upsampleFunction, from, to.Add(-interval)) // we want to query [from,to)
+		if err != nil {
+			return err
+		}
+		resampled = append(resampled, r)
+		iterations = r.Len()
+	}
+
+	for i := 0; i < iterations; i++ {
+		result := make([]eval.Result, 0, len(resampled))
+		var now time.Time
+		for _, series := range resampled {
+			snow := series.GetTime(i)
+			if !now.IsZero() && now != snow { // this should not happen because all series' belong to a single data frame
+				return errors.New("failed to resample input data. timestamps are not aligned")
+			}
+			now = snow
+			value := series.GetValue(i)
+			var state = eval.Normal
+			if value == nil {
+				continue
+			} else if *value != 0 {
+				state = eval.Alerting
+			}
+			result = append(result, eval.Result{
+				Instance: series.GetLabels(),
+				State:    state,
+				Results:  nil,
+				Values: map[string]eval.NumberValueCapture{
+					d.refID: {
+						Var:    d.refID,
+						Labels: series.GetLabels(),
+						Value:  value,
+					},
+				},
+				EvaluatedAt: now,
+			})
+		}
+		if len(result) == 0 {
+			result = append(result, eval.Result{
+				State:       eval.NoData,
+				EvaluatedAt: now,
+			})
+		}
+		err := callback(now, result)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}