counters: want to be able to count multiple fields of a variant

Sometimes a variant of an enum that derives Count has multiple fields which both can and should be counted --- for example, see https://github.com/oxidecomputer/hubris/pull/1742#discussion_r1556483876, where we have a variant like:

enum Trace {
    // ...
    I2cError { 
        txn: I2cTxn,
       #[count(children)] code: i2c::ResponseCode,
    }
    // ...
}

where I2cTxn is also an enum that derives Count. It would be nice to be able to generated a doubly-nested set of counters, so that we can count the I2C error code separately for each transaction type. Currently, the derive(Count) attribute can't do that. It would be nice if it could.

Start by reading the derive(Count) implementation and the #[count(children)] handling described in this issue, then review the linked pull-request discussion for the motivating I2cError shape. Determine how multiple counted fields should produce nested counters, and consider the work complete when an enum variant can count both its transaction and response-code fields separately.