test: add unit tests for Prometheus metrics endpoint

Author: kimdre

internal/prometheus/metrics_test.go

@@ -0,0 +1,53 @@
+package prometheus
+
+import (
+	"github.com/kimdre/doco-cd/internal/config"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// TestServe tests the metrics endpoint serving functionality.
+func TestServe(t *testing.T) {
+	expectedStatusCode := 200
+	expectedContentType := "text/plain; version=0.0.4; charset=utf-8; escaping=underscores"
+
+	appConfig, err := config.GetAppConfig()
+	if err != nil {
+		t.Fatalf("Failed to get app config: %v", err)
+	}
+
+	AppInfo.WithLabelValues("test", appConfig.LogLevel, time.Now().Format(time.RFC3339)).Set(1)
+
+	req, err := http.NewRequest("GET", MetricsPath, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rr := httptest.NewRecorder()
+
+	handler := promhttp.Handler()
+	handler.ServeHTTP(rr, req)
+
+	if status := rr.Code; status != expectedStatusCode {
+		t.Errorf("Expected status code %d, got %d", expectedStatusCode, status)
+	}
+
+	if contentType := rr.Header().Get("Content-Type"); contentType != expectedContentType {
+		t.Errorf("Expected Content-Type %s, got %s", expectedContentType, contentType)
+	}
+
+	// Check if the response body is not empty
+	if rr.Body.Len() == 0 {
+		t.Error("Expected non-empty response body, got empty")
+	}
+
+	// Check if the response body contains the expected metrics
+	if !strings.Contains(rr.Body.String(), "doco_cd_info") {
+		t.Error("Expected response body to contain 'doco_cd_info' metric, but it does not")
+	}
+}