[metadata] Don't ref entire read for unknown metadata added to hpack …

…table (grpc#34863)

src/core/lib/experiments/experiments.yaml

@@ -307,6 +307,10 @@
   expiry: 2024/02/01
   owner: [email protected]
   test_tags: [resource_quota_test]
+- name: uniquely_unowned
+  description: Ensure HPACK table takes a unique copy of data when parsing unknown metadata
+  expiry: 2024/03/03
+  owner: [email protected]
 - name: work_serializer_clears_time_cache
   description:
     Have the work serializer clear the time cache when it dispatches work.

src/core/lib/experiments/rollouts.yaml

@@ -134,6 +134,8 @@
   default: false
 - name: unconstrained_max_quota_buffer_size
   default: false
+- name: uniquely_unowned
+  default: true
 - name: work_serializer_clears_time_cache
   default: true
 - name: work_serializer_dispatch

src/core/lib/transport/metadata_batch.h

@@ -40,6 +40,7 @@
 #include <grpc/support/log.h>
 
 #include "src/core/lib/compression/compression_internal.h"
+#include "src/core/lib/experiments/experiments.h"
 #include "src/core/lib/gprpp/chunked_vector.h"
 #include "src/core/lib/gprpp/if_list.h"
 #include "src/core/lib/gprpp/packed_table.h"
@@ -647,7 +648,11 @@ class ParseHelper {
       absl::string_view key) {
     return ParsedMetadata<Container>(
         typename ParsedMetadata<Container>::FromSlicePair{},
-        Slice::FromCopiedString(key), std::move(value_), transport_size_);
+        Slice::FromCopiedString(key),
+        IsUniquelyUnownedEnabled() && will_keep_past_request_lifetime_
+            ? value_.TakeUniquelyOwned()
+            : std::move(value_),
+        transport_size_);
   }
 
  private: