Add event.outcome to metrics aggregation

[dgieselaar]

See elastic/apm#299

We'll be adding event.outcome to transaction docs, which (per https://www.elastic.co/guide/en/ecs/current/ecs-allowed-values-event-outcome.html) has a limited set of values: success, failure, unknown, (unset)

This field will be used for error rates. So that we can use aggregated metrics docs for the calculations, we will need to include this field in transaction metrics docs, which also implies that it needs to be included in the aggregation key. This should have no or little impact on series cardinality, as event.outcome will typically be subsumed by transaction.result.