Don't aggregate apdex to the service when the error rate and operation rate aren't being aggregated

This is a small change to make conditional aggregations a little more consistent.

At present, it's possible to exclude an SLI from a service aggregation. This is helpful for excluding, for example, Redis secondaries from the main Redis service aggregation.

At present, this has the effect of excluding operations rate and error rate (and thereby also error ratio), but apdex scores continue to be aggregated.

This change makes apdex scores consistent with errors and operation rates, and excludes them from the aggregation when the other two are.

This change does not have any impact at present, since none of the excluded SLIs have an apdex, however, we will be introducing other exclusions, such as for regional aggregations and having this consistency will be helpful for that change.