Add docstrings for CriticalPathInfo

Bug: 271526845
Test: n/a
Change-Id: I7ec5a9f7b39fd7441f1aac6556a77046a8210ae8
diff --git a/ui/metrics/metrics_proto/metrics.pb.go b/ui/metrics/metrics_proto/metrics.pb.go
index 5e144f4..6a664dc 100644
--- a/ui/metrics/metrics_proto/metrics.pb.go
+++ b/ui/metrics/metrics_proto/metrics.pb.go
@@ -1437,6 +1437,8 @@
 	return nil
 }
 
+// CriticalPathInfo contains critical path nodes's information.
+// A critical path is a path determining the minimum time needed for the whole build given perfect parallelism.
 type CriticalPathInfo struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache