Proposal: 'volume inspect' should show data usage

Description
For practical needs I propose the cli show the volume size on the default inspect format, so the user don't have to run

docker volume system df 

or

docker volume system df -v

To realize the current size of the running volume.

Just don't want to make a PR before asking. If approved I'll make the PR.

Actual behavior:

[
    {
        "CreatedAt": "2017-09-29T09:14:12-05:00",
        "Driver": "local",
        "Labels": {
            "com.docker.compose.project": "myApp",
            "com.docker.compose.volume": "db-postgres"
        },
        "Mountpoint": "/var/lib/docker/volumes/myApp_db-postgres/_data",
        "Name": "myApp_db-postgres",
        "Options": {},
        "Scope": "local"
    }
]

Proposal behavior:

[
    {
        "CreatedAt": "2017-09-29T09:14:12-05:00",
        "Driver": "local",
        "Labels": {
            "com.docker.compose.project": "myApp",
            "com.docker.compose.volume": "db-postgres"
        },
        "Mountpoint": "/var/lib/docker/volumes/myApp_db-postgres/_data",
        "Name": "myApp_db-postgres",
        "Options": {},
        "Size": "90MB",
        "Scope": "local"
    }
]

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:      17.07.0-ce-rc2
 API version:  1.31
 Go version:   go1.8.3
 Git commit:   36ce605
 Built:        Mon Aug  7 23:45:34 2017
 OS/Arch:      linux/amd64

Server:
 Version:      17.07.0-ce-rc2
 API version:  1.31 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   36ce605
 Built:        Mon Aug  7 23:46:55 2017
 OS/Arch:      linux/amd64
 Experimental: false

Output of docker info:

Containers: 10
 Running: 8
 Paused: 0
 Stopped: 2
Images: 68
Server Version: 17.07.0-ce-rc2
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins: 
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 3addd840653146c90a254301d6c3a663c7fd6429
runc version: 2d41c047c83e09a6d61d464906feb2a2f3c52aa4
init version: 949e6fa
Security Options:
 seccomp
  Profile: default
Kernel Version: 4.12.13-300.fc26.x86_64
Operating System: Fedora 26 (Workstation Edition)
OSType: linux
Architecture: x86_64
CPUs: 12
Total Memory: 15.59GiB
Name: Highwind
ID: 2WLY:Z6ID:XV65:VHRM:JBQR:W67K:R3JQ:ISDF:V5PY:CNRN:Q3VX:TSWF
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

Additional environment details (AWS, VirtualBox, physical, etc.):
physical

Start at the docker volume inspect CLI entry point and compare its current output with docker volume system df -v. Determine how volume data usage can be obtained for the inspect response; done means the default inspect output includes a size value like the proposed Size field.