[server] Stop exposing /metrics for self-hosted users (#2569)

Doesn't impact Ente's production instances since this endpoint was
disallowed on the load balancer, but instead of requiring the same for
folks who are self hosting we modify the source to not expose /metrics
on :8080 (they'll still be available on :2112).

Refs:
-
2199a42d96/middleware.go (L345)
- https://github.com/ente-io/ente/discussions/2562

server/cmd/museum/main.go

@@ -353,7 +353,7 @@ func main() {
 
 	p := ginprometheus.NewPrometheus("museum")
 	p.ReqCntURLLabelMappingFn = urlSanitizer
-	p.Use(server)
+	server.Use(p.HandlerFunc())
 
 	// note: the recover middleware must be in the last
 	server.Use(requestid.New(), middleware.Logger(urlSanitizer), cors(), gzip.Gzip(gzip.DefaultCompression), middleware.PanicRecover())