Alerting: Add traceability headers for alert queries (#57127)

* Define EvaluationContext

* Refactor ConditionEval to use new context struct

* Refactor QueriesAndExpressionsEval to use EvaluationContext

* Remove dead field from AlertExecCtx

* Refactor Validate to use EvaluationContext

* Get rid of privately used AlertExecCtx

* Move EvaluationContext to new file and add helper

* Add builder pattern and bind rule info to context

* Extract header logic and add rule UID header

* Fix missing call

pkg/services/ngalert/eval/context.go

@@ -0,0 +1,43 @@
+package eval
+
+import (
+	"context"
+	"time"
+
+	"github.com/grafana/grafana/pkg/services/ngalert/models"
+	"github.com/grafana/grafana/pkg/services/user"
+)
+
+// EvaluationContext represents the context in which a condition is evaluated.
+type EvaluationContext struct {
+	Ctx     context.Context
+	User    *user.SignedInUser
+	At      time.Time
+	RuleUID string
+}
+
+func Context(ctx context.Context, user *user.SignedInUser) EvaluationContext {
+	return EvaluationContext{
+		Ctx:  ctx,
+		User: user,
+		At:   time.Now(),
+	}
+}
+
+func (c EvaluationContext) When(t time.Time) EvaluationContext {
+	c.At = t
+	return c
+}
+
+func (c EvaluationContext) WithRule(r *models.AlertRule) EvaluationContext {
+	if r != nil {
+		c.RuleUID = r.UID
+	}
+	return c
+}
+
+func (c EvaluationContext) WithTimeout(timeout time.Duration) (EvaluationContext, context.CancelFunc) {
+	timeoutCtx, cancel := context.WithTimeout(c.Ctx, timeout)
+	c.Ctx = timeoutCtx
+	return c, cancel
+}