[registry][doc] Mark STRUCT related fields as Experimental

Mark STRUCT metric/report related fields as Experimental DO NOT USE to
avoid confusion.

Change-Id: I8c6130cad12db3aafa8edbdf80dd1e832be5282a
Reviewed-on: https://fuchsia-review.googlesource.com/c/cobalt/+/1426694
Reviewed-by: Alexandre Zani <azani@google.com>
Commit-Queue: Anivia Li <aniviali@google.com>
diff --git a/src/registry/metric_definition.proto b/src/registry/metric_definition.proto
index b5235af..4866932 100644
--- a/src/registry/metric_definition.proto
+++ b/src/registry/metric_definition.proto
@@ -233,6 +233,8 @@
   // These IDs must not be reused in new reports.
   repeated uint32 deleted_report_ids = 26;
 
+  // Experimental. DO NOT USE!
+  // A field in the STRUCT metric.
   message StructField {
     uint32 id = 1;
     string name = 2;
@@ -259,6 +261,8 @@
     string string_candidate_file = 5;
   }
 
+  // Experimental. DO NOT USE!
+  // A list of fields in the STRUCT metric.
   repeated StructField struct_fields = 27;
 }
 
diff --git a/src/registry/report_definition.proto b/src/registry/report_definition.proto
index 7ea26bf..76df5de 100644
--- a/src/registry/report_definition.proto
+++ b/src/registry/report_definition.proto
@@ -661,6 +661,8 @@
   // the system profile that is currently active on the device.
   bool expedited_sending = 29;
 
+  // Experimental. DO NOT USE!
+  // A specification of an aggregate for a STRUCT report.
   message AggregateDefinition {
     uint32 id = 1;
 
@@ -729,6 +731,8 @@
     string string_candidate_file = 8;
   }
 
+  // Experimental. DO NOT USE!
+  // A list of aggregates for a STRUCT report.
   repeated AggregateDefinition aggregates = 33;
 
   ///////////////////  Fields used by all report types ///////////////////