feat(metering): cache pending trie nodes for faster state root calculation

[niran]

Summary

Cache trie nodes computed during pending flashblock state root calculations and reuse them across bundle metering simulations. This optimization ensures that each bundle simulation only pays the I/O cost for its own state changes, not for all previous flashblock state changes.

Problem

Without caching, every bundle simulation must:

1. Compute the hashed post-state for all flashblock changes
2. Read trie nodes from disk for all accounts/storage touched by flashblocks
3. Then compute the bundle's incremental changes on top

With many bundles being simulated per flashblock, this redundant I/O becomes a bottleneck.

Solution

Introduce PendingTrieCache that:

1. Caches TrieUpdates and HashedPostState from pending state root computation
2. Uses arc_swap::ArcSwap for lock-free reads and atomic updates
3. Provides ensure_cached() for lazy computation and caching
4. Prepends cached data to bundle's TrieInput via prepend_cached()

Changes

• metrics.rs: New module with metrics for cache performance tracking.
• trie_cache.rs: New module implementing PendingTrieCache with ensure_cached() method that computes and caches the pending trie input, or returns the cached entry if it matches the current flashblock.
• meter.rs: Add PendingTrieInput struct containing TrieUpdates and HashedPostState. Add shared compute_pending_trie_input() helper with metrics. Update meter_bundle to use cached trie input when available. Use TrieInput::prepend_cached() to prepend pending trie for incremental state root calculation.
• rpc.rs: Integrate with PendingTrieCache to compute/retrieve cached trie data for bundle simulations.
• lib.rs: Export PendingTrieCache and PendingTrieInput.
• Cargo.toml: Add arc-swap, metrics, metrics-derive, and reth-trie-common dependencies.

Metrics

New Prometheus metrics for monitoring cache performance:

• reth_metering_pending_trie_cache_hits - counter for cache hits
• reth_metering_pending_trie_cache_misses - counter for cache misses (trie computed)
• reth_metering_pending_trie_compute_duration - histogram of compute time in seconds

Performance Impact

Before: Each bundle simulation reads trie nodes for all flashblock changes
After: Each bundle simulation only reads trie nodes for its own changes

The improvement scales with:

• Number of accounts/storage slots modified by flashblocks
• Number of bundles simulated per flashblock

Test Plan

• Existing metering tests pass (20 tests)
• State root calculation produces correct results with cached data
• cargo +nightly clippy passes
• cargo +nightly fmt passes

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 2/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[refcell]

Looks like this needs a rebase, but otherwise, core triedb cache changes look solid