Problem

Because the current implementation of KVStates is a dedicated VirtualMap, it is straightforward to implement Store.size() methods like this one as here:

    @Override
    public long sizeOfState() {
        return tokenState.size();
    }

As all state is consolidated into one VirtualMap (aka MegaMap), the effort of preserving the OnDiskKVState.size() method will no longer be a good cost/benefit ratio. This means Stores need a new implementation of their e.g. sizeOfState() methods.

Solution

• Add a new singleton ENTITY_COUNTS state in EntityIdService that has a field (e.g. uint64 num_accounts or uint64 num_bytecodes) for each type of entity.
• Add a 0.59-only step in SystemSetup.doPostUpgradeSetup() that initializes all these counts using direct calls to KVState.size() for every K/V state used by every service.
• Update the WritableEntityIdStore API to "vend" entity id numbers by type, so that each time a service wants to create a new entity of a certain type, the EntityIdService can increment the correct ENTITY_COUNTS field.
• Replace all Store.sizeOfState() implementations by getting a ReadableEntityIdStore and returning its e.g. numAccounts() or numBytecodes() getter as appropriate.
• Switch StoreMetrics.updateCount() callbacks to be done by a StateChangeListener with stateTypes() == Set.of(SINGLETON) instead of in OnDiskWritableKVState.
• Mark KVState.size() as deprecated and ensure there are no remaining non-test usages of this API.

Testing

• Unit tests
• Load a state and validate the counts are correctly updating in Entity_Counts map
• All existing tests should pass