Add monitoring data for metrics aggregation

[simitt]

From 8.7 onwards the APM Server creates service summary, service transaction, transaction and span destination metrics aggregations and also collects overflow information in an other bucket. The only aggregator that currently reports monitoring data about the aggregation is the tx metrics aggregator.

• All aggregators need to report metrics on overflow buckets and current number of active groups.
• The tx metrics aggregator reporting needs to be updated to include monitoring information about which group has overflowed (service, tx, per-service-tx).

This was an oversight to not be added during development and the fix should be backported to 8.7.