Delay DB size metric until `Db` is started

[MegaRedHand]

We have a race condition. If this measurement is called before Db is started, the measurement fails with the message:

Error when calling MFA defined by measurement: 'Elixir.LambdaEthereumConsensus.Telemetry' db_size []
Class=exit
Reason={noproc,{'Elixir.GenServer',call,
                                   ['Elixir.LambdaEthereumConsensus.Store.Db',
                                    get_ref,5000]}}