refactor: lazy get metadata

This changes how metadata handler works - now metadata will be fetched
only once during first request.

pkg/cmd/agent/metadata.go

@@ -5,6 +5,7 @@ package main
 import (
 	"context"
 	"net/http"
+	"sync"
 
 	"github.com/go-chi/render"
 	"github.com/pkg/errors"
@@ -14,21 +15,25 @@ import (
 )
 
 func newMetadataHandler(logger log.Logger) (http.HandlerFunc, error) {
-	ctx := context.Background()
 	metaSvc, err := cloudmeta.NewCloudMeta(logger)
 	if err != nil {
 		return nil, errors.Wrap(err, "NewCloudMeta")
 	}
 
-	instanceMeta, err := metaSvc.GetInstanceMetadata(ctx)
-	if err != nil {
-		// Metadata may not be available for several reasons:
-		// 1. running on-premise 2. disabled 3. smth went wrong with metadata server.
-		// As we cannot distinguish between these cases, we can only log err.
-		logger.Error(ctx, "GetInstanceMetadata", "err", err)
-	}
+	lazyGetMetadata := sync.OnceValues(func() (cloudmeta.InstanceMetadata, error) {
+		return metaSvc.GetInstanceMetadata(context.Background())
+	})
 
 	return func(w http.ResponseWriter, r *http.Request) {
+		instanceMeta, err := lazyGetMetadata()
+		if err != nil {
+			// Metadata may not be available for several reasons:
+			// 1. running on-premise 2. disabled 3. smth went wrong with metadata server.
+			// As we cannot distinguish between these cases, we can only log err.
+			logger.Error(r.Context(), "GetInstanceMetadata", "err", err)
+			render.Respond(w, r, models.InstanceMetadata{})
+			return
+		}
 		render.Respond(w, r, models.InstanceMetadata{
 			CloudProvider: string(instanceMeta.CloudProvider),
 			InstanceType:  instanceMeta.InstanceType,