Image metrics

[feiskyer]

Kubelet has already added image metrics API in CRI (see here), we should decide how to support it.

// StorageIdentifier uniquely identify the storage..
message StorageIdentifier{
    // UUID of the device.
    string uuid = 1;
}

// FilesystemUsage provides the filesystem usage information.
message FilesystemUsage {
    // Timestamp in nanoseconds at which the information were collected. Must be > 0.
    int64 timestamp = 1;
    // The underlying storage of the filesystem.
    StorageIdentifier storage_id = 2;
    // UsedBytes represents the bytes used for images on the filesystem.
    // This may differ from the total bytes used on the filesystem and may not
    // equal CapacityBytes - AvailableBytes.
    UInt64Value used_bytes = 3;
    // InodesUsed represents the inodes used by the images.
    // This may not equal InodesCapacity - InodesAvailable because the underlying
    // filesystem may also be used for purposes other than storing images.
    UInt64Value inodes_used = 4;
}

message ImageFsInfoResponse {
    // Information of image filesystem(s).
    repeated FilesystemUsage image_filesystems = 1;
}