Add structure documentation for metadata

Adds comment showing the structure of the buckets
as stored in boltdb. Includes all bucket names, key names
and descriptions of the stored values.

Moves the metadata comment to package level to be exposed
in the godoc overview.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/buckets.go

@@ -14,13 +14,11 @@
    limitations under the License.
 */
 
-package metadata
-
-import (
-	digest "github.com/opencontainers/go-digest"
-	bolt "go.etcd.io/bbolt"
-)
-
+// Package metadata stores all labels and object specific metadata by namespace.
+// This package also contains the main garbage collection logic  for cleaning up
+// resources consistently and atomically. Resources used by backends will be
+// tracked in the metadata store to be exposed to consumers of this package.
+//
 // The layout where a "/" delineates a bucket is described in the following
 // section. Please try to follow this as closely as possible when adding
 // functionality. We can bolster this with helpers and more structure if that
@@ -43,6 +41,84 @@ import (
 //
 // key: object-specific key identifying the storage bucket for the objects
 // contents.
+//
+// Below is the current database schema. This should be updated each time
+// the structure is changed in addition to adding a migration and incrementing
+// the database version. Note that `╘══*...*` refers to maps with arbitrary
+// keys.
+//  ├──version : <varint>                        - Latest version, see migrations
+//  └──v1                                        - Schema version bucket
+//     ╘══*namespace*
+//        ├──labels
+//        │  ╘══*key* : <string>                 - Label value
+//        ├──image
+//        │  ╘══*image name*
+//        │     ├──createdat : <binary time>     - Created at
+//        │     ├──updatedat : <binary time>     - Updated at
+//        │     ├──target
+//        │     │  ├──digest : <digest>          - Descriptor digest
+//        │     │  ├──mediatype : <string>       - Descriptor media type
+//        │     │  └──size : <varint>            - Descriptor size
+//        │     └──labels
+//        │        ╘══*key* : <string>           - Label value
+//        ├──containers
+//        │  ╘══*container id*
+//        │     ├──createdat : <binary time>     - Created at
+//        │     ├──updatedat : <binary time>     - Updated at
+//        │     ├──spec : <binary>               - Proto marshaled spec
+//        │     ├──image : <string>              - Image name
+//        │     ├──snapshotter : <string>        - Snapshotter name
+//        │     ├──snapshotKey : <string>        - Snapshot key
+//        │     ├──runtime
+//        │     │  ├──name : <string>            - Runtime name
+//        │     │  ├──extensions
+//        │     │  │  ╘══*name* : <binary>       - Proto marshaled extension
+//        │     │  └──options : <binary>         - Proto marshaled options
+//        │     └──labels
+//        │        ╘══*key* : <string>           - Label value
+//        ├──snapshots
+//        │  ╘══*snapshotter*
+//        │     ╘══*snapshot key*
+//        │        ├──name : <string>            - Snapshot name in backend
+//        │        ├──createdat : <binary time>  - Created at
+//        │        ├──updatedat : <binary time>  - Updated at
+//        │        ├──parent : <string>          - Parent snapshot name
+//        │        ├──children
+//        │        │  ╘══*snapshot key* : <nil>  - Child snapshot reference
+//        │        └──labels
+//        │           ╘══*key* : <string>        - Label value
+//        ├──content
+//        │  ├──blob
+//        │  │  ╘══*blob digest*
+//        │  │     ├──createdat : <binary time>  - Created at
+//        │  │     ├──updatedat : <binary time>  - Updated at
+//        │  │     ├──size : <varint>            - Blob size
+//        │  │     └──labels
+//        │  │        ╘══*key* : <string>        - Label value
+//        │  └──ingests
+//        │     ╘══*ingest reference*
+//        │        ├──ref : <string>             - Ingest reference in backend
+//        │        ├──expireat : <binary time>   - Time to expire ingest
+//        │        └──expected : <digest>        - Expected commit digest
+//        └──leases
+//           ╘══*lease id*
+//              ├──createdat : <binary time>     - Created at
+//              ├──labels
+//              │  ╘══*key* : <string>           - Label value
+//              ├──snapshots
+//              │  ╘══*snapshotter*
+//              │     ╘══*snapshot key* : <nil>  - Snapshot reference
+//              ├──content
+//              │  ╘══*blob digest* : <nil>      - Content blob reference
+//              └──ingests
+//                 ╘══*ingest reference* : <nil> - Content ingest reference
+package metadata
+
+import (
+	digest "github.com/opencontainers/go-digest"
+	bolt "go.etcd.io/bbolt"
+)
+
 var (
 	bucketKeyVersion          = []byte(schemaVersion)
 	bucketKeyDBVersion        = []byte("version")    // stores the version of the schema