Revisit default fields for aggregated transactions

Currently, the following fields are aggregated on for transaction metrics:

https://github.com/elastic/apm-server/blob/master/x-pack/apm-server/aggregation/txmetrics/aggregator.go#L423-L436:

```go
type transactionAggregationKey struct {
traceRoot bool
agentName string
containerID string
hostname string
kubernetesPodName string
serviceEnvironment string
serviceName string
serviceVersion string
transactionName string
transactionOutcome string
transactionResult string
transactionType string
}
```

Some of these fields were based on the presence of UI filters that needed them, but they've since been removed. We should look at the list to see if some fields can now be dropped, which could result in a better compression ratio.