Alerting: Tiny refactor on the eval and schedule packages (#66130)

* Alerting: Tiny refactor on the eval and schedule packages

two very small things:

- We had a constructor on something called a `Context` which is not a `context.Context` so let's just name that constructor `NewContext`
- The user that we use to run query evaluations is the same (with some variation) abstract it to a function so that it can be re-used when necessary.

* Update pkg/services/ngalert/schedule/schedule.go

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

* Update pkg/services/ngalert/schedule/schedule.go

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

---------

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

pkg/services/ngalert/schedule/schedule.go

@@ -373,21 +373,7 @@ func (sch *schedule) ruleRoutine(grafanaCtx context.Context, key ngmodels.AlertR
 		logger := logger.New("version", e.rule.Version, "attempt", attempt, "now", e.scheduledAt)
 		start := sch.clock.Now()
 
-		schedulerUser := &user.SignedInUser{
-			UserID:           -1,
-			IsServiceAccount: true,
-			Login:            "grafana_scheduler",
-			OrgID:            e.rule.OrgID,
-			OrgRole:          org.RoleAdmin,
-			Permissions: map[int64]map[string][]string{
-				e.rule.OrgID: {
-					datasources.ActionQuery: []string{
-						datasources.ScopeAll,
-					},
-				},
-			},
-		}
-		evalCtx := eval.Context(ctx, schedulerUser)
+		evalCtx := eval.NewContext(ctx, SchedulerUserFor(e.rule.OrgID))
 		ruleEval, err := sch.evaluatorFactory.Create(evalCtx, e.rule.GetEvalCondition())
 		var results eval.Results
 		var dur time.Duration
@@ -580,3 +566,20 @@ func (sch *schedule) getRuleExtraLabels(evalCtx *evaluation) map[string]string {
 	}
 	return extraLabels
 }
+
+func SchedulerUserFor(orgID int64) *user.SignedInUser {
+	return &user.SignedInUser{
+		UserID:           -1,
+		IsServiceAccount: true,
+		Login:            "grafana_scheduler",
+		OrgID:            orgID,
+		OrgRole:          org.RoleAdmin,
+		Permissions: map[int64]map[string][]string{
+			orgID: {
+				datasources.ActionQuery: []string{
+					datasources.ScopeAll,
+				},
+			},
+		},
+	}
+}