Better estimate for aggregated events count

## Motivation

The current algorithm calculates events total for the whole `CombinedMetrics`. This approach works well as long as we need to merge forward. However, when we need to split or partition `CombinedMetrics` then we can no longer estimate the events accurately.

## Solution

Track events total for each aggregate (including overflow) rather than for the whole `CombinedMetrics`. This will require more storage but we will be able to track accurate events total for splits or partitions. Extra storage cost based on current limits will be:

`(max_span_groups (40_000) + max_txn_groups (40_000) + max_svc_txn_groups (8_000) + overflows_per_svc (8_000) + overflow_global (1)) * (events_total_size_tracked_by_float64) 8 bytes = ~770KB`

The extra storage will be used by pebble.