Looks good, though I've got one more test for you to enhance/add.

PS I appreciate the use of a very unattractive name for this proto.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @itsbilal, @petermattis, and @sumeerbhola)

pkg/storage/enginepb/mvcc.proto, line 89 at r1 (raw file):

  option (gogoproto.goproto_stringer) = false;
  option (gogoproto.equal) = true;
  option (gogoproto.populate) = true;

I believe this is only used by kv/kvserver/below_raft_protos_test.go. It would be nice to verify that the serialization of this proto doesn't accidentally changed, because the serialization happens below Raft. But I'm not sure if you can hook directly into the below_raft_protos_test.go due to the way those golden files are checked that they are still being marshaled below Raft. If you can hook into that test, great. If you can't, I'd suggest forking the relevant portions (i.e. randomly populating a proto using a fixed seed and verifying the output when serialized).

TFTR!
Modified below_raft_protos_test.go

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pkg/storage/sst_writer_test.go, line 148 at r2 (raw file):

	require.Equal(t, string(sstRocks), string(sstPebble))
	itRocks.Close()
	itPebble.Close()

this is unrelated -- the test was failing with a pebble: cache has non-zero reference count error. Not sure why it has not been consistently failing.

pkg/storage/enginepb/mvcc.proto, line 89 at r1 (raw file):

You were right -- running the kvserver tests with COCKROACH_STORAGE_ENGINE=pebble fails with
*enginepb.MVCCMetadataSubsetForMergeSerialization: missing fixture! Please adjust belowRaftGoldenProtos if necessary

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @itsbilal and @sumeerbhola)

pkg/storage/sst_writer_test.go, line 148 at r2 (raw file):

GC needs to fire in order for the finalizer-based leak detection to trigger. I wouldn't be surprised if there are one or two more such places lurking. It has been straightforward to track down the missing Close calls in my experience.