feat(evm): standardize slow block JSON output for cross-client metrics

[CPerezz]

Summary

Implement standardized JSON format for slow block logging to enable cross-client performance analysis and protocol research.

This change is part of the Cross-Client Execution Metrics initiative proposed by Gary Rong and CPerezz.

Motivation

Standardized execution metrics are critical for:

• Cross-client performance comparison
• Network health monitoring
• Data-driven protocol research

Real-world example: The EIP-7907 analysis used execution metrics to measure code read latency, per-call overhead scaling, and block execution breakdown. Without standardized metrics across clients, such analysis cannot be validated cross-client.

References

• Original proposal: https://hackmd.io/dg7rizTyTXuCf2LSa2LsyQ (Gary Rong)
• EIP-7907 analysis: https://ethresear.ch/t/data-driven-analysis-on-eip-7907/23850

JSON Format

{
  "level": "warn",
  "msg": "Slow block",
  "block": { "number": ..., "hash": ..., "gas_used": ..., "tx_count": ... },
  "timing": { "execution_ms": ..., "total_ms": ... },
  "throughput": { "mgas_per_sec": ... },
  "state_reads": { "accounts": ..., "storage_slots": ..., "code": ..., "code_bytes": ... },
  "state_writes": { "accounts": ..., "storage_slots": ... },
  "cache": {
    "account": { "hits": ..., "misses": ..., "hit_rate": ... },
    "storage": { ... },
    "code": { ... }
  }
}

Example trace:

{
    "timestamp": "2026-01-25T12:33:51.268394Z",
    "level": "WARN",
    "fields": {
        "message": "Slow block",
        "block.number": 9,
        "block.hash": "0x02e95075ddbb6866370968d92014b789f5917c05784e6091217a6a22e234be86",
        "block.gas_used": 105393,
        "block.tx_count": 1,
        "timing.execution_ms": "0.063",
        "timing.state_read_ms": "0.006",
        "timing.state_hash_ms": "0.465",
        "timing.commit_ms": "73.504",
        "timing.total_ms": "74.039",
        "throughput.mgas_per_sec": "1671.79",
        "state_reads.accounts": 7,
        "state_reads.storage_slots": 0,
        "state_reads.code": 0,
        "state_reads.code_bytes": 0,
        "state_writes.accounts": 3,
        "state_writes.accounts_deleted": 0,
        "state_writes.storage_slots": 0,
        "state_writes.storage_slots_deleted": 0,
        "state_writes.code": 1,
        "state_writes.code_bytes": 240,
        "state_writes.eip7702_delegations_set": 0,
        "state_writes.eip7702_delegations_cleared": 0,
        "cache.account.hits": 32,
        "cache.account.misses": 52,
        "cache.account.hit_rate": "38.10",
        "cache.storage.hits": 0,
        "cache.storage.misses": 0,
        "cache.storage.hit_rate": "0.00",
        "cache.code.hits": 0,
        "cache.code.misses": 0,
        "cache.code.hit_rate": "0.00"
    },
    "target": "reth::slow_block"
}

[shekhirin]

Thank you for the initial implementation, taking this over here #21433