[java-storage] Getting both Metadata and Content with a Single request

I might be wrong but looking at our distributed traces, it seems following code will send two GET (JSON API) requests to `stogare.googleapis.com`

```
Blob blob = storage.get(blobId);
byte[] content = blob.getContent();
```

Is there a way to get both the content and (some) metadata with a single request? There's `Storage.readAllBytes` but it does not return metadata.
We are specifically interested in `Content-Encoding`, `Content-Type` and a few other standard headers (`Cache-Control`, `Content-Length`, etc).

We have millions of small objects, and it'd be great if we can cut both the latency and the cost by half.

首先检查 Java Storage.get(blobId) 和 Storage.readAllBytes API，以及 issue 中描述的底层 JSON API 请求。确定单个请求是否可以同时返回内容以及 Content-Encoding、Content-Type、Cache-Control 和 Content-Length。对于所请求的元数据，如果受支持的 API 行为已通过单个请求完成文档记录或实现并经过验证，则视为完成。

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