gRPC FieldMask not rendered correctly in Swagger Page

### What version of gRPC and what language are you using?

Microsoft.AspNetCore.Grpc.Swagger v 0.8.11
C#

### What operating system (Linux, Windows,...) and version?

Windows 10

### What runtime / compiler are you using (e.g. .NET Core SDK version `dotnet --info`)

MSVC 17.9.6 / net 8.04

### What did you do?
Updated Microsoft.AspNetCore.Grpc.Swagger from 0.3.4 to 0.8.11

### What did you expect to see?
Swagger page allowing me to add repeated paths to a fieldmask.
Rendering an 'Add String Value' button and a removal option:


### What did you see instead?
Swagger page allowing only one path.

The option to 'Add String Value' has gone:


### Example .proto:
```
// The request message
message EntityRequest {
string EntityTypeCode = 1;
string SearchCriteria = 2;
bool ActiveOnly = 3;
google.protobuf.FieldMask mask = 4;
bool ExplicitPathing = 5;
}
```

If I create my own FieldMask type, it all starts to work again:

```
message FieldMask2 {
repeated string paths = 1;
}

...
bool ActiveOnly = 3;
FieldMask2 mask = 4;
bool ExplicitPathing = 5;
...
```
Any idea why it doesn't like the regular FieldMask?