runtime/metrics: fix Read panic on empty slice

Add a length check in metrics.Read to handle empty slices gracefully.
Previously, passing an empty slice would panic due to accessing &m[0]
on a zero-length slice.

The fix ensures Read is a no-op for empty inputs, which is the
expected behavior.

Fixes #77231

src/runtime/metrics/description_test.go

@@ -156,3 +156,10 @@ func TestDocs(t *testing.T) {
 		fmt.Fprintf(os.Stderr, "go test -generate: doc.go already up-to-date\n")
 	}
 }
+
+func TestReadEmptySlice(t *testing.T) {
+	// Test that Read does not panic when given an empty slice.
+	// This should be a no-op.
+	metrics.Read(nil)
+	metrics.Read([]metrics.Sample{})
+}

src/runtime/metrics/sample.go

@@ -43,5 +43,8 @@ func runtime_readMetrics(unsafe.Pointer, int, int)
 // Sample values with names not appearing in [All] will have their Value populated
 // as KindBad to indicate that the name is unknown.
 func Read(m []Sample) {
+	if len(m) == 0 {
+		return
+	}
 	runtime_readMetrics(unsafe.Pointer(&m[0]), len(m), cap(m))
 }