feat(alerting): Add ilert alert provider (#1082)

* feat(alerting): Add ilert alert provider

* fix: removed additional endpoint from Readme

* fix: removed newline

Co-authored-by: TwiN <twin@linux.com>

* fix: formatting

* fix: restAPIUrl typo

* fix: removed type field, adjusted tests

---------

Co-authored-by: TwiN <twin@linux.com>

alerting/provider/ilert/ilert.go

@@ -0,0 +1,168 @@
+package ilert
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/TwiN/gatus/v5/alerting/alert"
+	"github.com/TwiN/gatus/v5/client"
+	"github.com/TwiN/gatus/v5/config/endpoint"
+	"gopkg.in/yaml.v3"
+)
+
+const (
+	restAPIUrl = "https://api.ilert.com/api/v1/events/gatus/"
+)
+
+var (
+	ErrIntegrationKeyNotSet   = errors.New("integration key is not set")
+	ErrDuplicateGroupOverride = errors.New("duplicate group override")
+)
+
+type Config struct {
+	IntegrationKey string `yaml:"integration-key"`
+}
+
+func (cfg *Config) Validate() error {
+	if len(cfg.IntegrationKey) == 0 {
+		return ErrIntegrationKeyNotSet
+	}
+	return nil
+}
+
+func (cfg *Config) Merge(override *Config) {
+	if len(override.IntegrationKey) > 0 {
+		cfg.IntegrationKey = override.IntegrationKey
+	}
+}
+
+// AlertProvider is the configuration necessary for sending an alert using ilert
+type AlertProvider struct {
+	DefaultConfig Config `yaml:",inline"`
+
+	// DefaultAlert is the default alert configuration to use for endpoints with an alert of the appropriate type
+	DefaultAlert *alert.Alert `yaml:"default-alert,omitempty"`
+
+	// Overrides is a list of Override that may be prioritized over the default configuration
+	Overrides []Override `yaml:"overrides,omitempty"`
+}
+
+type Override struct {
+	Group  string `yaml:"group"`
+	Config `yaml:",inline"`
+}
+
+func (provider *AlertProvider) Validate() error {
+	registeredGroups := make(map[string]bool)
+	if provider.Overrides != nil {
+		for _, override := range provider.Overrides {
+			if isAlreadyRegistered := registeredGroups[override.Group]; isAlreadyRegistered || override.Group == "" {
+				return ErrDuplicateGroupOverride
+			}
+			registeredGroups[override.Group] = true
+		}
+	}
+	return provider.DefaultConfig.Validate()
+}
+
+func (provider *AlertProvider) Send(ep *endpoint.Endpoint, alert *alert.Alert, result *endpoint.Result, resolved bool) error {
+	cfg, err := provider.GetConfig(ep.Group, alert)
+	if err != nil {
+		return err
+	}
+	buffer := bytes.NewBuffer(provider.buildRequestBody(cfg, ep, alert, result, resolved))
+
+	req, err := http.NewRequest(http.MethodPost, fmt.Sprintf("%s%s", restAPIUrl, cfg.IntegrationKey), buffer)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	response, err := client.GetHTTPClient(nil).Do(req)
+	if err != nil {
+		return err
+	}
+	defer response.Body.Close()
+	if response.StatusCode > 399 {
+		body, _ := io.ReadAll(response.Body)
+		return fmt.Errorf("call to provider alert returned status code %d: %s", response.StatusCode, string(body))
+	}
+
+	return err
+}
+
+type Body struct {
+	Alert            alert.Alert                 `json:"alert"`
+	Name             string                      `json:"name"`
+	Group            string                      `json:"group"`
+	Status           string                      `json:"status"`
+	Title            string                      `json:"title"`
+	Details          string                      `json:"details,omitempty"`
+	ConditionResults []*endpoint.ConditionResult `json:"condition_results"`
+	URL              string                      `json:"url"`
+}
+
+func (provider *AlertProvider) buildRequestBody(cfg *Config, ep *endpoint.Endpoint, alert *alert.Alert, result *endpoint.Result, resolved bool) []byte {
+	var details, status string
+	if resolved {
+		status = "resolved"
+	} else {
+		status = "firing"
+	}
+
+	if len(alert.GetDescription()) > 0 {
+		details = alert.GetDescription()
+	} else {
+		details = "No description"
+	}
+
+	var body []byte
+	body, _ = json.Marshal(Body{
+		Alert:            *alert,
+		Name:             ep.Name,
+		Group:            ep.Group,
+		Title:            ep.DisplayName(),
+		Status:           status,
+		Details:          details,
+		ConditionResults: result.ConditionResults,
+		URL:              ep.URL,
+	})
+	return body
+}
+
+func (provider *AlertProvider) GetDefaultAlert() *alert.Alert {
+	return provider.DefaultAlert
+}
+
+func (provider *AlertProvider) GetConfig(group string, alert *alert.Alert) (*Config, error) {
+	cfg := provider.DefaultConfig
+	// Handle group overrides
+	if provider.Overrides != nil {
+		for _, override := range provider.Overrides {
+			if group == override.Group {
+				cfg.Merge(&override.Config)
+				break
+			}
+		}
+	}
+	// Handle alert overrides
+	if len(alert.ProviderOverride) != 0 {
+		overrideConfig := Config{}
+		if err := yaml.Unmarshal(alert.ProviderOverrideAsBytes(), &overrideConfig); err != nil {
+			return nil, err
+		}
+		cfg.Merge(&overrideConfig)
+	}
+	// Validate the configuration
+	err := cfg.Validate()
+	return &cfg, err
+}
+
+// ValidateOverrides validates the alert's provider override and, if present, the group override
+func (provider *AlertProvider) ValidateOverrides(group string, alert *alert.Alert) error {
+	_, err := provider.GetConfig(group, alert)
+	return err
+}