This PR introduces tracking for entity count changes and current storage size of entities in BlockStateMetrics.

While the tracking mechanism follows a similar pattern to other metrics (gas, op, read_bytes, write_bytes), there's an important distinction in how the newly introduced metrics entity_changes and storage_size values are recorded:

• The other metrics use monotonic counters that accumulate totals over time (e.g., total bytes read)
• The new entity_count_changes metric tracks the net change in entity counts per block (e.g., +5 id_1 added, -2 id_2 removed)
• The new storage_size metric tracks the actual current size of stored entities as a live readout. If the entity increases/reduces in size due to a write event, then this metric will reflect the new size of the entity after the write event.

For entity_count_changes, each block's measurement represents changes that occurred within that block, not cumulative totals or absolute counts. To determine the total number of entities at a given block, these changes would need to be summed across all previous blocks.