[storage] add metric "lastest_transaction_version"
[storage] add metric "lastest_transaction_version"

On our overview dashboard we use the commited_txns counter to track num of txns in total, which is not guaranteed to be precise (when for example prometheus clears data).

Instead, adding a prometheus gause to reflect the latest version directly.

libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra