Add a test for `Notify` of the Discord integration (#4082)

Just to ensure this works correclty as expected, I originally thought there was a bug with the shadowing of the `content` varible but there isn't - to avoid further confusion I have followed up on this document left by George: https://github.com/prometheus/alertmanager/pull/3555#discussion_r1398448423

Signed-off-by: gotjosh <josue.abreu@gmail.com>

notify/discord/discord.go

@@ -137,11 +137,11 @@ func (n *Notifier) Notify(ctx context.Context, as ...*types.Alert) (bool, error)
 	if n.conf.WebhookURL != nil {
 		url = n.conf.WebhookURL.String()
 	} else {
-		content, err := os.ReadFile(n.conf.WebhookURLFile)
+		b, err := os.ReadFile(n.conf.WebhookURLFile)
 		if err != nil {
 			return false, fmt.Errorf("read webhook_url_file: %w", err)
 		}
-		url = strings.TrimSpace(string(content))
+		url = strings.TrimSpace(string(b))
 	}
 
 	w := webhook{

notify/discord/discord_test.go

@@ -17,6 +17,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -168,3 +169,63 @@ func TestDiscordReadingURLFromFile(t *testing.T) {
 
 	test.AssertNotifyLeaksNoSecret(ctx, t, notifier, u.String())
 }
+
+func TestDiscord_Notify(t *testing.T) {
+	// Create a fake HTTP server to simulate the Discord webhook
+	var resp string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Read the request as a string
+		body, err := io.ReadAll(r.Body)
+		require.NoError(t, err, "reading request body failed")
+		// Store the request body in the response
+		resp = string(body)
+
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Create a temporary file to simulate the WebhookURLFile
+	tempFile, err := os.CreateTemp("", "webhook_url")
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		require.NoError(t, os.Remove(tempFile.Name()))
+	})
+
+	// Write the fake webhook URL to the temp file
+	_, err = tempFile.WriteString(srv.URL)
+	require.NoError(t, err)
+
+	// Create a DiscordConfig with the WebhookURLFile set
+	cfg := &config.DiscordConfig{
+		WebhookURLFile: tempFile.Name(),
+		HTTPConfig:     &commoncfg.HTTPClientConfig{},
+		Title:          "Test Title",
+		Message:        "Test Message",
+		Content:        "Test Content",
+	}
+
+	// Create a new Discord notifier
+	notifier, err := New(cfg, test.CreateTmpl(t), log.NewNopLogger())
+	require.NoError(t, err)
+
+	// Create a context and alerts
+	ctx := context.Background()
+	ctx = notify.WithGroupKey(ctx, "1")
+	alerts := []*types.Alert{
+		{
+			Alert: model.Alert{
+				Labels: model.LabelSet{
+					"lbl1": "val1",
+				},
+				StartsAt: time.Now(),
+				EndsAt:   time.Now().Add(time.Hour),
+			},
+		},
+	}
+
+	// Call the Notify method
+	ok, err := notifier.Notify(ctx, alerts...)
+	require.NoError(t, err)
+	require.False(t, ok)
+
+	require.Equal(t, "{\"content\":\"Test Content\",\"embeds\":[{\"title\":\"Test Title\",\"description\":\"Test Message\",\"color\":10038562}]}\n", resp)
+}