Generates and returns an estimate of how much gas is necessary to allow the transaction to complete. The transaction will not be added to the blockchain.
See eth_call parameters, expect that all properties are optional. If no gas limit is specified Geth uses the block gas limit from the pending block as an upper bound. As a result the returned estimate might not be enough to execute the call/transaction when the amount of gas is higher than the pending block gas limit.
QUANTITY(hex string) - the number of gas used.