Add connection webhook notifications (#392)

Also 

* Added decode of LoginStart message
* Add metrics backend constants
* Updated usage section
* Documented MaxFrameLength

server/webhook_notifier.go

@@ -0,0 +1,148 @@
+package server
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"github.com/sirupsen/logrus"
+	"log"
+	"net"
+	"net/http"
+	"time"
+)
+
+// WebhookNotifier implements ConnectionNotifier by sending a POST request to a webhook URL.
+// The payload is a JSON object defined by WebhookNotifierPayload.
+type WebhookNotifier struct {
+	url         string
+	requireUser bool
+
+	client *http.Client
+}
+
+const (
+	WebhookEventConnecting = "connect"
+)
+
+const (
+	WebhookStatusMissingBackend          = "missing-backend"
+	WebhookStatusFailedBackendConnection = "failed-backend-connection"
+	WebhookStatusSuccess                 = "success"
+)
+
+type WebhookNotifierPayload struct {
+	Event           string      `json:"event"`
+	Timestamp       time.Time   `json:"timestamp"`
+	Status          string      `json:"status"`
+	Client          *ClientInfo `json:"client"`
+	Server          string      `json:"server"`
+	PlayerInfo      *PlayerInfo `json:"player,omitempty"`
+	BackendHostPort string      `json:"backend,omitempty"`
+	Error           string      `json:"error,omitempty"`
+}
+
+func NewWebhookNotifier(url string, requireUser bool) *WebhookNotifier {
+
+	return &WebhookNotifier{
+		url:         url,
+		requireUser: requireUser,
+		client: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+func (w *WebhookNotifier) NotifyMissingBackend(ctx context.Context, clientAddr net.Addr, server string, playerInfo *PlayerInfo) error {
+	if w.requireUser && playerInfo == nil {
+		return nil
+	}
+
+	payload := &WebhookNotifierPayload{
+		Event:      WebhookEventConnecting,
+		Timestamp:  time.Now(),
+		Status:     WebhookStatusMissingBackend,
+		Client:     ClientInfoFromAddr(clientAddr),
+		Server:     server,
+		PlayerInfo: playerInfo,
+		Error:      "No backend found",
+	}
+
+	return w.send(ctx, payload)
+}
+
+func (w *WebhookNotifier) NotifyFailedBackendConnection(ctx context.Context, clientAddr net.Addr, server string,
+	playerInfo *PlayerInfo, backendHostPort string, err error) error {
+	if w.requireUser && playerInfo == nil {
+		return nil
+	}
+
+	payload := &WebhookNotifierPayload{
+		Event:           WebhookEventConnecting,
+		Timestamp:       time.Now(),
+		Status:          WebhookStatusFailedBackendConnection,
+		Client:          ClientInfoFromAddr(clientAddr),
+		Server:          server,
+		PlayerInfo:      playerInfo,
+		BackendHostPort: backendHostPort,
+		Error:           err.Error(),
+	}
+
+	return w.send(ctx, payload)
+}
+
+func (w *WebhookNotifier) NotifyConnected(ctx context.Context, clientAddr net.Addr, serverAddress string, playerInfo *PlayerInfo, backendHostPort string) error {
+	if w.requireUser && playerInfo == nil {
+		return nil
+	}
+
+	payload := &WebhookNotifierPayload{
+		Event:           WebhookEventConnecting,
+		Timestamp:       time.Now(),
+		Status:          WebhookStatusSuccess,
+		Client:          ClientInfoFromAddr(clientAddr),
+		Server:          serverAddress,
+		PlayerInfo:      playerInfo,
+		BackendHostPort: backendHostPort,
+	}
+
+	return w.send(ctx, payload)
+}
+
+func (w *WebhookNotifier) send(ctx context.Context, payload *WebhookNotifierPayload) error {
+	jsonPayload, err := json.Marshal(payload)
+	if err != nil {
+		return fmt.Errorf("failed to marshal webhook payload: %v", err)
+	}
+
+	req, err := http.NewRequestWithContext(
+		ctx,
+		http.MethodPost,
+		w.url,
+		bytes.NewBuffer(jsonPayload),
+	)
+	if err != nil {
+		return fmt.Errorf("failed to create webhook request: %v", err)
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+
+	go func() {
+		resp, err := w.client.Do(req)
+		if err != nil {
+			// Handle error
+			log.Printf("Failed to send webhook notification: %v", err)
+			return
+		}
+		_ = resp.Body.Close()
+
+		if resp.StatusCode >= 400 {
+			logrus.
+				WithField("status", resp.StatusCode).
+				Warn("webhook receiver responded with an error")
+		}
+
+	}()
+
+	return nil
+}