[storage] add counters for db iter/get latency & bandwidth
[storage] add counters for db iter/get latency & bandwidth

Closes: #4919
Approved by: andll

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