feat(dashboard): add block transaction count and SR set change panels

Add two new panel sections to the java-tron-server Grafana dashboard
to support metrics from tronprotocol/java-tron#6624:

- Block Metrics: empty block rate, per-SR empty block count,
  avg transactions per block, transaction count distribution
- SR Set Change: event table and cumulative totals by action

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: warku123