Allow firebaseStorageDownloadTokens to be set as a SettableMetadata

[jbryanh]

I'm not sure if this is a bug or should remain a simple feature request. Because of the error I received, I think putting this as a feature request is appropriate. There is a dearth of documentation on this functionality.

When I set the following code:

​final metadata = firebase_storage.SettableMetadata( ​customMetadata: {'firebaseStorageDownloadTokens': customToken});

_uploadTask = _storage.ref().child(filePath).putFile(widget.file!, metadata);

I get the following error:
E/StorageException( 6906): Caused by: java.io.IOException: { "error": { "code": 400, "message": "Not allowed to set custom metadata for firebaseStorageDownloadTokens" }}

So, the enhancement request is either:

1. enable this.
2. Provide suitable documentation for a person to know how to properly set the token.

(incidentally, I have to run on Android to get an error, iOS gives no error, as documented here:

Extended errors below:
E/StorageException( 6906): StorageException has occurred.
E/StorageException( 6906): An unknown error occurred, please check the HTTP result code and inner exception for server response.
E/StorageException( 6906): Code: -13000 HttpResult: 400
E/StorageException( 6906): The server has terminated the upload session
E/StorageException( 6906): java.io.IOException: The server has terminated the upload session
E/StorageException( 6906): at com.google.firebase.storage.UploadTask.serverStateValid(UploadTask.java:339)
E/StorageException( 6906): at com.google.firebase.storage.UploadTask.shouldContinue(UploadTask.java:308)
E/StorageException( 6906): at com.google.firebase.storage.UploadTask.run(UploadTask.java:232)
E/StorageException( 6906): at com.google.firebase.storage.StorageTask.lambda$getRunnable$7$StorageTask(StorageTask.java:1072)
E/StorageException( 6906): at com.google.firebase.storage.-$$Lambda$StorageTask$_IQT0HwL0SAn4-mUxblkILcDlBo.run(Unknown Source:2)
E/StorageException( 6906): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
E/StorageException( 6906): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
E/StorageException( 6906): at java.lang.Thread.run(Thread.java:923)
E/StorageException( 6906): Caused by: java.io.IOException: { "error": { "code": 400, "message": "Not allowed to set custom metadata for firebaseStorageDownloadTokens" }}
E/StorageException( 6906): at com.google.firebase.storage.network.NetworkRequest.parseResponse(NetworkRequest.java:445)
E/StorageException( 6906): at com.google.firebase.storage.network.NetworkRequest.parseErrorResponse(NetworkRequest.java:462)
E/StorageException( 6906): at com.google.firebase.storage.network.NetworkRequest.processResponseStream(NetworkRequest.java:453)
E/StorageException( 6906): at com.google.firebase.storage.network.NetworkRequest.performRequest(NetworkRequest.java:272)
E/StorageException( 6906): at com.google.firebase.storage.network.NetworkRequest.performRequest(NetworkRequest.java:289)
E/StorageException( 6906): at com.google.firebase.storage.UploadTask.send(UploadTask.java:469)
E/StorageException( 6906): at com.google.firebase.storage.UploadTask.uploadChunk(UploadTask.java:428)
E/StorageException( 6906): at com.google.firebase.storage.UploadTask.run(UploadTask.java:231)
E/StorageException( 6906): ... 5 more

[pr-Mais]

Refer to my answer here: #7340 (comment)