Add aggregate configuration support and update documentation

- Introduced AggregateConfig to manage aggregation settings in the gateway configuration.
- Added validation for reserved alias 'agg' and included tests for aggregate alias handling.
- Updated config.example.yaml to demonstrate aggregate configuration options.
- Enhanced README.md to include information about the new aggregation endpoint and its usage.
- Modified gateway.go to integrate the new aggregate handler for processing aggregation requests.

internal/aggregate/handlers_test.go

@@ -0,0 +1,75 @@
+package aggregate
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/telemt/telemt-api/internal/config"
+)
+
+func TestHandlerResolveAndFetch(t *testing.T) {
+	up := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/v1/stats/users" {
+			http.NotFound(w, r)
+			return
+		}
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"ok":       true,
+			"data":     []map[string]any{{"username": "u1", "total_octets": 42, "current_connections": 0}},
+			"revision": "abc",
+		})
+	}))
+	defer up.Close()
+
+	cfg := &config.Config{
+		Servers: []config.Server{
+			{Alias: "test", BaseURL: up.URL, PathPrefix: "/v1"},
+		},
+	}
+	if err := cfg.Validate(); err != nil {
+		t.Fatal(err)
+	}
+	parsed, err := cfg.Parse()
+	if err != nil {
+		t.Fatal(err)
+	}
+	h := NewHandler(parsed, up.Client())
+	req := httptest.NewRequest(http.MethodGet, "/api/agg/summary?aliases=test", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status %d body %s", rec.Code, rec.Body.String())
+	}
+	var env struct {
+		OK   bool           `json:"ok"`
+		Data SummaryData    `json:"data"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &env); err != nil {
+		t.Fatal(err)
+	}
+	if !env.OK || env.Data.FleetTotalOctets != 42 {
+		t.Fatalf("data: %+v", env.Data)
+	}
+}
+
+func TestHandlerMethodNotAllowed(t *testing.T) {
+	cfg := &config.Config{
+		Servers: []config.Server{{Alias: "x", BaseURL: "http://127.0.0.1:1", PathPrefix: "/v1"}},
+	}
+	if err := cfg.Validate(); err != nil {
+		t.Fatal(err)
+	}
+	parsed, err := cfg.Parse()
+	if err != nil {
+		t.Fatal(err)
+	}
+	h := NewHandler(parsed, http.DefaultClient)
+	req := httptest.NewRequest(http.MethodPost, "/api/agg/summary", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != http.StatusMethodNotAllowed {
+		t.Fatalf("got %d", rec.Code)
+	}
+}