ipfs.metrics.scope error -> TODO: mock repo

[sanderpick]

When adding files, I see this with the new App based IPFS setup:

08:47:14.800 ERROR       core: failure on stop:  context canceled; TODO: mock repo builder.go:36

This doesn't seem to cause any other issues... just a noisy error in the logs. The context is always related to "ipfs.metrics.scope":

[sanderpick]

Actually, this is happening because I'm using the HashOnly setting when adding data at one point during the sync process. This causes this mock repo to be constructed: https://github.com/ipfs/go-ipfs/blob/master/core/coreapi/unixfs.go#L64. When it's no longer needed, I'm guessing Close is called on it, which returns this TODO err to the mock node's error channel.

Still harmless, just good to know the actual cause. We should probably PR to go-ipfs a change that either removes the error or otherwise makes it more obvious to users who just want to use the hash only option of add.