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/server/gateway.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 
+	"github.com/telemt/telemt-api/internal/aggregate"
 	"github.com/telemt/telemt-api/internal/config"
 	"github.com/telemt/telemt-api/internal/proxy"
 )
@@ -21,6 +22,7 @@ import (
 type Gateway struct {
 	parsed        *config.Parsed
 	proxies       map[string]*httputil.ReverseProxy
+	agg           *aggregate.Handler
 	log           *slog.Logger
 	transport     *http.Transport
 	promHandler   http.Handler
@@ -63,6 +65,7 @@ func NewGateway(p *config.Parsed, log *slog.Logger) (*Gateway, error) {
 		}
 		g.proxies[s.Alias] = rp
 	}
+	g.agg = aggregate.NewHandler(p, &http.Client{Transport: t})
 	return g, nil
 }
 
@@ -170,6 +173,10 @@ func (g *Gateway) serve(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	const prefix = "/api/"
+	if r.URL.Path == "/api/agg" || strings.HasPrefix(r.URL.Path, "/api/agg/") {
+		g.agg.ServeHTTP(w, r)
+		return
+	}
 	if !strings.HasPrefix(r.URL.Path, prefix) {
 		http.NotFound(w, r)
 		return