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/fetch.go

@@ -0,0 +1,148 @@
+package aggregate
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+
+	"github.com/telemt/telemt-api/internal/config"
+)
+
+const statsUsersPath = "stats/users"
+
+// FetchStatsUsers calls GET {base}{path_prefix}/stats/users for each alias.
+func FetchStatsUsers(ctx context.Context, client *http.Client, parsed *config.Parsed, aliases []string) []ServerFetchResult {
+	out := make([]ServerFetchResult, 0, len(aliases))
+	for _, alias := range aliases {
+		srv := parsed.ByAlias[alias]
+		if srv == nil {
+			out = append(out, ServerFetchResult{
+				Alias: alias,
+				OK:    false,
+				Error: "unknown alias",
+			})
+			continue
+		}
+		u, err := url.Parse(srv.BaseURL)
+		if err != nil {
+			out = append(out, ServerFetchResult{
+				Alias: alias,
+				OK:    false,
+				Error: err.Error(),
+			})
+			continue
+		}
+		target := joinPathPrefix(u, srv.PathPrefix, statsUsersPath)
+		start := time.Now()
+		req, err := http.NewRequestWithContext(ctx, http.MethodGet, target.String(), nil)
+		if err != nil {
+			out = append(out, ServerFetchResult{
+				Alias:     alias,
+				OK:        false,
+				LatencyMs: time.Since(start).Milliseconds(),
+				Error:     err.Error(),
+			})
+			continue
+		}
+		if auth := parsed.AuthByAlias[alias]; auth != "" {
+			req.Header.Set("Authorization", auth)
+		}
+		resp, err := client.Do(req)
+		latency := time.Since(start).Milliseconds()
+		if err != nil {
+			out = append(out, ServerFetchResult{
+				Alias:     alias,
+				OK:        false,
+				LatencyMs: latency,
+				Error:     err.Error(),
+			})
+			continue
+		}
+		body, readErr := io.ReadAll(io.LimitReader(resp.Body, 64<<20))
+		_ = resp.Body.Close()
+		if readErr != nil {
+			out = append(out, ServerFetchResult{
+				Alias:      alias,
+				OK:         false,
+				HTTPStatus: resp.StatusCode,
+				LatencyMs:  latency,
+				Error:      readErr.Error(),
+			})
+			continue
+		}
+		if resp.StatusCode != http.StatusOK {
+			out = append(out, ServerFetchResult{
+				Alias:      alias,
+				OK:         false,
+				HTTPStatus: resp.StatusCode,
+				LatencyMs:  latency,
+				Error:      fmt.Sprintf("http %d: %s", resp.StatusCode, truncate(string(body), 200)),
+			})
+			continue
+		}
+		var env statsUsersEnvelope
+		if err := json.Unmarshal(body, &env); err != nil {
+			out = append(out, ServerFetchResult{
+				Alias:      alias,
+				OK:         false,
+				HTTPStatus: resp.StatusCode,
+				LatencyMs:  latency,
+				Error:      "invalid json: " + err.Error(),
+			})
+			continue
+		}
+		if !env.OK {
+			out = append(out, ServerFetchResult{
+				Alias:      alias,
+				OK:         false,
+				HTTPStatus: resp.StatusCode,
+				LatencyMs:  latency,
+				Error:      "upstream ok=false",
+			})
+			continue
+		}
+		out = append(out, ServerFetchResult{
+			Alias:      alias,
+			OK:         true,
+			HTTPStatus: resp.StatusCode,
+			LatencyMs:  latency,
+			Revision:   env.Revision,
+			Users:      env.Data,
+		})
+	}
+	return out
+}
+
+func joinPathPrefix(base *url.URL, pathPrefix, rest string) *url.URL {
+	rel := strings.Trim(pathPrefix, "/")
+	if rest != "" {
+		if rel != "" {
+			rel = rel + "/" + rest
+		} else {
+			rel = rest
+		}
+	}
+	var parts []string
+	for _, seg := range strings.Split(rel, "/") {
+		if seg != "" {
+			parts = append(parts, seg)
+		}
+	}
+	if len(parts) == 0 {
+		out := *base
+		return &out
+	}
+	return base.JoinPath(parts...)
+}
+
+func truncate(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	return s[:n] + "…"
+}