metrics: instrument flashblock build and wait-for-next-tick by index

[teddyknox]

Summary

• Add per-flashblock-index histogram for build duration (op_rbuilder_flashblock_build_duration_by_index)
• Add per-flashblock-index histogram for wait-to-next-tick slack (op_rbuilder_flashblock_wait_for_next_tick_duration)

Test plan

• cargo check -p op-rbuilder passes
• Verify metrics are emitted correctly in a test environment

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds additional flashblock-related metrics to improve observability by tracking build latency and emission counts segmented by flashblock index.

Changes:

• Record a per-flashblock-index histogram for flashblock build duration (op_rbuilder_flashblock_build_duration_seconds_by_index).
• Record a per-flashblock-index counter for emitted flashblocks (op_rbuilder_flashblock_emitted_total).

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

crates/op-rbuilder/src/metrics.rs:87

• The PR description does not mention the addition of wait-for-next-tick metrics, but the code adds three new wait-related metrics: flashblock_wait_for_next_tick_duration (histogram), flashblock_wait_for_next_tick_gauge (gauge), and op_rbuilder_flashblock_wait_for_next_tick_duration_by_index (histogram). Consider updating the PR description to accurately reflect all metrics being added.

    /// Histogram of the time taken to sync a Flashblock
    pub flashblock_sync_duration: Histogram,
    /// Flashblock UTF8 payload byte size histogram
    pub flashblock_byte_size_histogram: Histogram,

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[SozinM]

@teddyknox reth recently merged changes to metrics paradigmxyz/reth#20232
Now we could create histogram with custom buckets (and labels)
Maybe this could be used to improve this metrics