Expressions: Move GEL into core as expressions (#29072)

* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins

pkg/expr/mathexp/reduce.go

@@ -0,0 +1,101 @@
+package mathexp
+
+import (
+	"fmt"
+	"math"
+
+	"github.com/grafana/grafana-plugin-sdk-go/data"
+)
+
+func Sum(v *data.Field) *float64 {
+	var sum float64
+	for i := 0; i < v.Len(); i++ {
+		if f, ok := v.At(i).(*float64); ok {
+			if f == nil {
+				nan := math.NaN()
+				return &nan
+			}
+			sum += *f
+		}
+	}
+	return &sum
+}
+
+func Avg(v *data.Field) *float64 {
+	sum := Sum(v)
+	f := *sum / float64(v.Len())
+	return &f
+}
+
+func Min(fv *data.Field) *float64 {
+	var f float64
+	if fv.Len() == 0 {
+		nan := math.NaN()
+		return &nan
+	}
+	for i := 0; i < fv.Len(); i++ {
+		if v, ok := fv.At(i).(*float64); ok {
+			if v == nil {
+				nan := math.NaN()
+				return &nan
+			}
+			if i == 0 || *v < f {
+				f = *v
+			}
+		}
+	}
+	return &f
+}
+
+func Max(fv *data.Field) *float64 {
+	var f float64
+	if fv.Len() == 0 {
+		nan := math.NaN()
+		return &nan
+	}
+	for i := 0; i < fv.Len(); i++ {
+		if v, ok := fv.At(i).(*float64); ok {
+			if v == nil {
+				nan := math.NaN()
+				return &nan
+			}
+			if i == 0 || *v > f {
+				f = *v
+			}
+		}
+	}
+	return &f
+}
+
+func Count(fv *data.Field) *float64 {
+	f := float64(fv.Len())
+	return &f
+}
+
+// Reduce turns the Series into a Number based on the given reduction function
+func (s Series) Reduce(rFunc string) (Number, error) {
+	var l data.Labels
+	if s.GetLabels() != nil {
+		l = s.GetLabels().Copy()
+	}
+	number := NewNumber(fmt.Sprintf("%v_%v", rFunc, s.GetName()), l)
+	var f *float64
+	fVec := s.Frame.Fields[1]
+	switch rFunc {
+	case "sum":
+		f = Sum(fVec)
+	case "mean":
+		f = Avg(fVec)
+	case "min":
+		f = Min(fVec)
+	case "max":
+		f = Max(fVec)
+	case "count":
+		f = Count(fVec)
+	default:
+		return number, fmt.Errorf("reduction %v not implemented", rFunc)
+	}
+	number.SetValue(f)
+
+	return number, nil
+}