How to specify type with metadata of pubsub?

The CloudEvent spec says we can specify the `type` in the metadata. The SDK allows metadata customization when publishing an event as below
```java
Map metadata = new HashMap<>();
metadata.put("type", "com.test.user.created");
Mono result = client.publishEvent("pubsub", "inbox", payload, metadata);
result.block(Duration.ofSeconds(2));
```

But the `type` seems always be `com.dapr.event.sent`. I debug into the gRPC code down to protocol buffer`serializeStringMapTo` and I am pretty sure the `type` is correctly written to the sidecar via 50001.

Are there anything wrong here? And can I specify other metadata like `metadata.rawPayload`?

If I do a POST with curl, this works
```
curl -X POST http://myurl... -d {"specversion" : "1.0", "type" : "com.mytest", "source" : "testcloudeventspubsub", "subject" : "Cloud Events Test", "id" : "111111", "time" : "2021-08-02T09:00:00Z", "datacontenttype" : "application/cloudevents+json", "data" : {"orderId": "100"}}
```
So I guess this problem only occurs in gRPC. However, the SDK does not support HTTP for pub/sub, right?

Start at the Java SDK's publishEvent path and the gRPC serializeStringMapTo call described in the issue; reproduce the metadata type case against the sidecar on port 50001 and compare it with the working HTTP request. Done means determining whether gRPC preserves CloudEvent metadata and documenting or correcting the observed behavior, including the metadata.rawPayload question.

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