JuiceFS gateway support for object metadata

[stevapple]

What would you like to be added:
Add support for storing, fetching and modifying object metadata in JuiceFS gateway.

Why is this needed:
Object metadata is used for statistics and will be user-facing (as HTTP headers), compared to user tags. This is extremely useful when we want to expose some custom metadata to downstream users (eg. web clients).

[davies]

Can you provide a more detailed example on how this feature will be used?

[stevapple]

@davies Object metadata is tightly associated with the object, and is user-facing. If we want to allow end-users to provide and get metadata through a public URL (pre-signed or anonymous), this is the only choice. We’re especially using this feature to add MIME and other useful information for media/file management backed by S3.

[prOOrc]

We have encountered an issue because of rclone tries to store mtime in the object metadata.

[zhijian-pro]

@prOOrc Can you provide the specific details for reproducing the error, we will not store the metadata, but it should not cause an error.

[zhijian-pro]

@stevapple We have no support plan for object metadata at this time.
Supporting the object metadata can lead to degraded performance in getObject operations.

[prOOrc]

@zhijian-pro Yes, it doesn't cause an error. But rclone tries to preserve the mtime in metadata. We have to ignore the modification time during file synchronization from/to sources that support the mtime.
I added metadata support in my fork. It can be enabled optionally.

[davies]

@prOOrc Can you send a PR for it?