feat: experiments logging improvements (#2049)

* feat: warn when enabling inactive experiments

* feat: TASK_ environment prefix

* feat: calculate experiment enabled/active instead of storing

* refactor: rename GetTaskVar to GetTaskEnv

* feat: experiments tests

internal/experiments/experiment.go

@@ -0,0 +1,56 @@
+package experiments
+
+import (
+	"fmt"
+	"slices"
+)
+
+type Experiment struct {
+	Name          string   // The name of the experiment.
+	AllowedValues []string // The values that can enable this experiment.
+	Value         string   // The version of the experiment that is enabled.
+}
+
+// New creates a new experiment with the given name and sets the values that can
+// enable it.
+func New(xName string, allowedValues ...string) Experiment {
+	value := getEnv(xName)
+	x := Experiment{
+		Name:          xName,
+		AllowedValues: allowedValues,
+		Value:         value,
+	}
+	xList = append(xList, x)
+	return x
+}
+
+func (x *Experiment) Enabled() bool {
+	return slices.Contains(x.AllowedValues, x.Value)
+}
+
+func (x *Experiment) Active() bool {
+	return len(x.AllowedValues) > 0
+}
+
+func (x Experiment) Valid() error {
+	if !x.Active() && x.Value != "" {
+		return &InactiveError{
+			Name: x.Name,
+		}
+	}
+	if !x.Enabled() && x.Value != "" {
+		return &InvalidValueError{
+			Name:          x.Name,
+			AllowedValues: x.AllowedValues,
+			Value:         x.Value,
+		}
+	}
+	return nil
+}
+
+func (x Experiment) String() string {
+	if x.Enabled() {
+		return fmt.Sprintf("on (%s)", x.Value)
+	}
+	return "off"
+}