feat: add mock telemetry server for local development (#21932)

Adds a standalone command that acts as a mock telemetry server,
receiving snapshots and printing them as a JSON stream to stdout. Useful
for local development testing with scripts/develop.sh by setting
CODER_TELEMETRY_ENABLE and CODER_TELEMETRY_URL environment variabless.

scripts/telemetry-server/main.go

@@ -0,0 +1,74 @@
+// telemetry-server is a standalone HTTP server that receives telemetry
+// snapshots and prints them as a JSON stream to stdout. This is useful for
+// local development. Test with scripts/develop.sh by setting:
+//
+//	CODER_TELEMETRY_ENABLE=true CODER_TELEMETRY_URL=http://127.0.0.1:8081
+//
+// Usage:
+//
+//	go run ./scripts/telemetry-server [--port 8081]
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"os"
+	"os/signal"
+	"time"
+)
+
+func main() {
+	port := flag.String("port", "8081", "Port to listen on")
+	flag.Parse()
+
+	enc := json.NewEncoder(os.Stdout)
+
+	mux := http.NewServeMux()
+
+	handleTelemetry := func(telemetryType string) http.HandlerFunc {
+		return func(w http.ResponseWriter, r *http.Request) {
+			body, err := io.ReadAll(r.Body)
+			if err != nil {
+				http.Error(w, err.Error(), http.StatusBadRequest)
+				return
+			}
+
+			output := map[string]any{
+				"type":    telemetryType,
+				"version": r.Header.Get("X-Telemetry-Version"),
+				"data":    json.RawMessage(body),
+			}
+			_ = enc.Encode(output)
+
+			w.WriteHeader(http.StatusAccepted)
+		}
+	}
+
+	mux.HandleFunc("POST /snapshot", handleTelemetry("snapshot"))
+	mux.HandleFunc("POST /deployment", handleTelemetry("deployment"))
+
+	addr := net.JoinHostPort("127.0.0.1", *port)
+	server := &http.Server{
+		Addr:              addr,
+		Handler:           mux,
+		ReadHeaderTimeout: 10 * time.Second,
+	}
+
+	go func() {
+		c := make(chan os.Signal, 1)
+		signal.Notify(c, os.Interrupt)
+		<-c
+		_ = server.Close()
+	}()
+
+	_, _ = fmt.Fprintf(os.Stdout, "Mock telemetry server listening on %s\n", addr)
+	if err := server.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+		_, _ = fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
+	}
+}