support/datastore: datastore provides retrieval of metrics on its ledger data

[sreuland]

What problem does your feature solve?

No visibility on status of exported data on Datastore, such as the smallest or greatest ledger sequence available on the datastore.

What would you like to see?

new method Datastore.GetMetrics returns a:

DataStoreMetrics {
   MinLedgerSequence: unit32 
   MaxLedgerSequence: uint32
}

using a struct so can scale to more meta information later.

What alternatives are there?

clients can manually try different bounded ranges with BufferedStorageBackend to explore what data is there, but that is not efficient or deterministic and not recommended.