datastore: document metadata operations

[karalabe]

There are quite a lot of metadata operations doable (kind, stat, etc retrieval), which is possible currently too, but only if you know the internal data types and related entity kinds.

E.g.

type KindStat struct {
    KindName            string `datastore:"kind_name"`
    EntityBytes         int    `datastore:"entity_bytes"`
    BuiltinIndexBytes   int    `datastore:"builtin_index_bytes"`
    BuiltinIndexCount   int    `datastore:"builtin_index_count"`
    CompositeIndexBytes int    `datastore:"composite_index_bytes"`
    CompositeIndexCount int    `datastore:"composite_index_count"`

    Count     int       `datastore:"count"`
    Bytes     int       `datastore:"bytes"`
    Timestamp time.Time `datastore:"timestamp"`
}

stats := []KindStat{}
datastore.NewQuery("__Stat_Kind__").GetAll(ctx, &stats)

It would be nice to collect all these metadata types, queries, operations, and have them under a more pleasant API :)

[perry-birch]

+1 Meta queries would be very helpful in the native libs. (Thanks for the struct def btw, exactly what I needed)

[jba]

I think we can document these in the package somewhere (perhaps by including the above struct). I don't think more client surface is needed.

[jba]

Moved to https://github.com/GoogleCloudPlatform/google-cloud-go/wiki/Feature-Backlog.