fix: stringify pyrra_window expression

While trying to use [Azure Prometheus Rules Converter](https://github.com/Azure/prometheus-collector/tree/main/tools/az-prom-rules-converter#az-prom-rules-converter) with generic rules enabled, the following error was thrown:

```
Failed to validate Prometheus Rules Schema
[
  {
    "instancePath": "/groups/2/rules/1/expr",
    "schemaPath": "#/properties/expr/type",
    "keyword": "type",
    "params": {
      "type": "string"
    },
    "message": "must be string"
  },
]
```

This corresponds to the `pyrra_window` expression, which is currently a raw integer. Turning it into a string fixes the issue and does not seem to have any impact on the rule on Prometheus. This is also in line with the fact that `pyrra_objective` is also stringified.

slo/rules.go

@@ -1160,7 +1160,7 @@ func (o Objective) GenericRules() (monitoringv1.RuleGroup, error) {
 	})
 	rules = append(rules, monitoringv1.Rule{
 		Record: "pyrra_window",
-		Expr:   intstr.FromInt(int(time.Duration(o.Window).Seconds())),
+		Expr:   intstr.FromString(strconv.FormatInt(int64(time.Duration(o.Window).Seconds()), 10)),
 		Labels: ruleLabels,
 	})
 

slo/rules_test.go

@@ -1,6 +1,7 @@
 package slo
 
 import (
+	"strconv"
 	"testing"
 	"time"
 
@@ -1689,7 +1690,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "monitoring-http-errors"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(int((28 * 24 * time.Hour).Seconds())),
+				Expr:   intstr.FromString(strconv.FormatInt(int64((28 * 24 * time.Hour).Seconds()), 10)),
 				Labels: map[string]string{"slo": "monitoring-http-errors"},
 			}, {
 				Record: "pyrra_availability",
@@ -1725,7 +1726,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "monitoring-grpc-errors"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(int((28 * 24 * time.Hour).Seconds())),
+				Expr:   intstr.FromString(strconv.FormatInt(int64((28 * 24 * time.Hour).Seconds()), 10)),
 				Labels: map[string]string{"slo": "monitoring-grpc-errors"},
 			}, {
 				Record: "pyrra_availability",
@@ -1757,7 +1758,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "monitoring-http-latency"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(2419200),
+				Expr:   intstr.FromString("2419200"),
 				Labels: map[string]string{"slo": "monitoring-http-latency"},
 			}, {
 				Record: "pyrra_availability",
@@ -1793,7 +1794,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "monitoring-grpc-latency"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(604800),
+				Expr:   intstr.FromString("604800"),
 				Labels: map[string]string{"slo": "monitoring-grpc-latency"},
 			}, {
 				Record: "pyrra_availability",
@@ -1825,7 +1826,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "monitoring-prometheus-operator-errors"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(1209600),
+				Expr:   intstr.FromString("1209600"),
 				Labels: map[string]string{"slo": "monitoring-prometheus-operator-errors"},
 			}, {
 				Record: "pyrra_availability",
@@ -1857,7 +1858,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "apiserver-write-response-errors"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(int((14 * 24 * time.Hour).Seconds())),
+				Expr:   intstr.FromString(strconv.FormatInt(int64((14 * 24 * time.Hour).Seconds()), 10)),
 				Labels: map[string]string{"slo": "apiserver-write-response-errors"},
 			}, {
 				Record: "pyrra_availability",
@@ -1889,7 +1890,7 @@ func TestObjective_GrafanaRules(t *testing.T) {
 				Labels: map[string]string{"slo": "up-targets"},
 			}, {
 				Record: "pyrra_window",
-				Expr:   intstr.FromInt(int((28 * 24 * time.Hour).Seconds())),
+				Expr:   intstr.FromString(strconv.FormatInt(int64((28 * 24 * time.Hour).Seconds()), 10)),
 				Labels: map[string]string{"slo": "up-targets"},
 			}, {
 				Record: "pyrra_availability",