fix(gcp): merge instead of clobber when setting instance labels

Instance.SetLabelsWithClient sent the caller's labels map directly to the
Compute API, which replaces all labels server-side. This is the same bug
class as the SetMetadata clobber regression already fixed in v1; calling
SetLabels to add a single label silently destroys every other label on
the instance.

Add a mergeLabels helper paralleling NewMetadata at line 524 and route
SetLabelsWithClient through it so existing labels are preserved.
TestSetLabelsWithClientMergesExisting verifies the merge.

OSS-3455.

Author: james00012

modules/gcp/compute.go

@@ -426,11 +426,14 @@ func (i *Instance) SetLabelsContextE(t testing.TestingT, ctx context.Context, la
 }
 
 // SetLabelsWithClient adds the tags to the given Compute Instance using the supplied *compute.Service.
+// New labels are merged into the instance's existing labels; passing a key that is already set
+// overwrites it, but other existing labels are preserved.
 // Prefer this variant in unit tests where the service is backed by an httptest fake server
 // (see compute_unit_test.go for the pattern).
 // The ctx parameter supports cancellation and timeouts.
 func (i *Instance) SetLabelsWithClient(ctx context.Context, service *compute.Service, labels map[string]string) error {
-	req := compute.InstancesSetLabelsRequest{Labels: labels, LabelFingerprint: i.LabelFingerprint}
+	merged := mergeLabels(i.Labels, labels)
+	req := compute.InstancesSetLabelsRequest{Labels: merged, LabelFingerprint: i.LabelFingerprint}
 
 	if _, err := service.Instances.SetLabels(i.projectID, ZoneURLToZone(i.Zone), i.Name, &req).Context(ctx).Do(); err != nil {
 		return fmt.Errorf("Instances.SetLabels(%s) got error: %w", i.Name, err)
@@ -439,6 +442,21 @@ func (i *Instance) SetLabelsWithClient(ctx context.Context, service *compute.Ser
 	return nil
 }
 
+// mergeLabels merges new key-value pairs into existing labels, preserving any keys not in the new set.
+// Keys present in newLabels overwrite the existing values.
+func mergeLabels(existing, newLabels map[string]string) map[string]string {
+	merged := make(map[string]string, len(existing)+len(newLabels))
+	for k, v := range existing {
+		merged[k] = v
+	}
+
+	for k, v := range newLabels {
+		merged[k] = v
+	}
+
+	return merged
+}
+
 // GetMetadata gets the given Compute Instance's metadata.
 //
 // Deprecated: Use [Instance.GetMetadataContext] instead.

modules/gcp/compute_test.go

@@ -2,6 +2,7 @@ package gcp_test
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
@@ -194,6 +195,40 @@ func TestSetLabelsWithClient(t *testing.T) {
 	require.NoError(t, inst.SetLabelsWithClient(context.Background(), svc, map[string]string{"env": "unit"}))
 }
 
+// TestSetLabelsWithClientMergesExisting — regression test for the SetLabels-clobbers-existing bug.
+// The instance already carries a label; SetLabels should merge the new label in, not drop the
+// existing one.
+func TestSetLabelsWithClientMergesExisting(t *testing.T) {
+	t.Parallel()
+
+	zoneURL := "https://www.googleapis.com/compute/v1/projects/p/zones/us-central1-a"
+	inst := fetchInstanceForTest(t, "p", "i", zoneURL)
+	inst.Labels = map[string]string{"team": "platform"}
+
+	var sentLabels map[string]string
+
+	handler := func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodPost, r.Method)
+		assert.Contains(t, r.URL.Path, "/instances/i/setLabels")
+
+		var req compute.InstancesSetLabelsRequest
+
+		require.NoError(t, json.NewDecoder(r.Body).Decode(&req))
+
+		sentLabels = req.Labels
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`{"name":"op","status":"DONE"}`))
+	}
+
+	svc := newFakeComputeService(t, http.HandlerFunc(handler))
+
+	require.NoError(t, inst.SetLabelsWithClient(context.Background(), svc, map[string]string{"env": "unit"}))
+
+	assert.Equal(t, "platform", sentLabels["team"], "existing label should be preserved")
+	assert.Equal(t, "unit", sentLabels["env"], "new label should be set")
+}
+
 func TestSetMetadataWithClient(t *testing.T) {
 	t.Parallel()