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

[warku123]

What does this PR do?

Add two new panel sections to the java-tron-server Grafana dashboard template to visualize the metrics introduced in tronprotocol/java-tron#6624:

1. Block Metrics row (4 panels):

   • Empty Block Rate — ratio of empty blocks over total blocks (timeseries)
   • Per-SR Empty Block Count (1h) — empty blocks per SR in the last hour (barchart)
   • Avg Transactions per Block — average transaction count per block (timeseries)
   • Block Transaction Count Distribution — non-cumulative histogram bucket breakdown as stacked bars (timeseries)

2. SR Set Change row (2 panels):

   • SR Set Change Events — table of add/remove events with color-coded action labels
   • SR Set Change Total — cumulative stat for SR additions and removals with threshold coloring

Why are these changes required?

tronprotocol/java-tron#6624 introduces tron:block_transaction_count (histogram) and tron:sr_set_change (counter) metrics. This PR updates the official Grafana dashboard template so node operators can import and use the new panels directly, reducing onboarding effort as suggested in this comment.

This PR has been tested by:

• Manual Testing (2-node private net with Prometheus + Grafana)

Follow up

• Depends on feat(metrics): add block transaction count and SR set change monitoring java-tron#6624 being merged (new metrics must exist for panels to populate data)

Extra details

• New panels are inserted as collapsed rows between "Block" and "Net" sections
• All PromQL queries follow the existing dashboard conventions ($group, $instance, backtick syntax, $__interval)
• No changes to existing panels; safe to merge independently (panels show "No data" until java-tron metrics are available)