message names with mutliple constants don't generate correctly when used in grpc

Given a message such as this:
```proto
message IDMessage {
string id = 1;
}
```

When it is used in an rpc as th einput message, this is generated
```python
async def get_revision(self) -> "Revision":
"""GetRevision returns a revision"""

request = IdMessage()

return await self._unary_unary(
"/somerpc.Name/GetRevision", request, Revision
)
```
if changed to:
```proto
message IdMessage {
string id = 1;
}
```
The generated prototype is then fixed:
```python
async def get_revision(self, *, id: str = "") ->"Revision":
"""GetRevision returns a revision"""

request = IdMessage()
request.id = id

return await self._unary_unary(
"/somerpc.Name/GetRevision", request, Revision
)
```

Reproduce the issue with the two protobuf message definitions and an RPC using the message as its input. Start by tracing the generated asynchronous gRPC client method and compare the multi-constant and single-constant cases. Done means both cases generate the expected method signature and request-field assignments.

Written by the indexing model from the issue text.