generate vtproto marshalers for filer_pb and use them on the metadata log path (#10337)

* generate vtproto marshalers for filer_pb and use them on the metadata log path

Reflection-based proto.Unmarshal allocates a fresh message tree through
reflect.New on every call. On the metadata subscription fan-out the same
event is decoded once per subscriber, so reflect.New tops the decode
churn under many mounts.

Generate MarshalVT/UnmarshalVT/SizeVT for filer.proto (a separate
filer_vtproto.pb.go, filer.pb.go untouched) and call them on the log
entry marshal and the subscribe/replay decode paths. UnmarshalVT
allocates message structs directly and copies byte and string fields, so
it stays wire-compatible with proto.Unmarshal and preserves the
non-aliasing the persisted-log cache depends on.

For SubscribeMetadataResponse this cuts decode allocations 69 -> 50 and
~4.5us -> ~2.1us per event; the win scales with subscriber overlap.

* marshal log entries directly into the buffer

SizeVT is allocation-free and MarshalToSizedBufferVT writes into a
pre-sized slice, so the log entry can be marshaled straight into
logBuffer.buf. This drops the per-entry MarshalVT allocation and the
follow-up copy on the write path.

* expand vtproto benchmarks: marshal, decode, and marshal-into-buffer by chunk count

Parametrize by nested-message count (chunks per event) and add encode +
zero-alloc marshal-into-buffer benchmarks alongside the decode one, so
the write-path win from MarshalToSizedBufferVT is measurable too.

* keep proto.Unmarshal for metadata events to preserve UTF-8 validation

UnmarshalVT skips proto3's UTF-8 validation of string fields, so a
SubscribeMetadataResponse with an invalid-UTF-8 string (e.g. Directory
"\xff") that proto.Unmarshal rejects would decode and reach path
filtering and subscribers. Decode events with proto.Unmarshal again;
UnmarshalVT stays on the log entry paths, whose only variable-length
fields are bytes and so carry no UTF-8 constraint.

Tests cover the codec difference and that a malformed event is skipped
before delivery.

weed/server/filer_grpc_server_sub_meta.go

@@ -515,6 +515,9 @@ func eachLogEntryFn(req *filer_pb.SubscribeMetadataRequest, sender metadataStrea
 			}
 		}
 		event := &filer_pb.SubscribeMetadataResponse{}
+		// proto.Unmarshal (not UnmarshalVT) validates UTF-8 in string fields, so
+		// malformed metadata is rejected here instead of reaching path filtering
+		// and subscribers.
 		if err := proto.Unmarshal(logEntry.Data, event); err != nil {
 			glog.Errorf("unexpected unmarshal filer_pb.SubscribeMetadataResponse: %v", err)
 			return false, fmt.Errorf("unexpected unmarshal filer_pb.SubscribeMetadataResponse: %w", err)