public OptionalLong blockNumber() {
    return getBlockHeadersData().blockNumber;
  }
Returns the block number that the message requests or {@link OptionalLong#empty()} if the
request specifies a block hash.

@return Block Number Requested or {@link OptionalLong#empty()}
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/messages/GetBlockHeadersMessage.java