Add latency metrics to assembler

[moradna]

#414

[Copilot]

Pull request overview

This PR extends the Assembler’s observability by adding histogram-based latency metrics around key batch-processing stages (fetching batches, waiting for batches after attestations, and appending batches to the ledger), and wires those metrics through the Assembler components and documentation.

Changes:

• Add three new Assembler histogram metrics: batch fetch latency, BA→batch availability latency, and batch ledger append latency.
• Instrument Prefetcher and Collator to observe the new latency metrics at the relevant points in the processing flow.
• Update unit tests/benchmarks and the metrics documentation to reflect the new metrics and constructor signatures.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
node/assembler/prefetcher.go	Injects Metrics into the Prefetcher and records batch fetch latency observations.
node/assembler/prefetcher_test.go	Updates Prefetcher tests to construct and pass Assembler metrics.
node/assembler/prefetch_benchmark/prefetch_benchmark_test.go	Updates benchmark setup to construct and pass Assembler metrics.
node/assembler/mocks/prefetcher_factory.go	Regenerates/updates the PrefetcherFactory fake to include the new Metrics parameter.
node/assembler/metrics.go	Defines new histogram metrics, initializes them, and includes their averages in periodic/stop logging.
node/assembler/collator.go	Records BA→batch latency and batch ledger append latency around the corresponding operations.
node/assembler/collator_test.go	Updates Collator test construction to include Metrics.
node/assembler/assembler.go	Wires Assembler metrics into the PrefetcherFactory and Collator initialization.
node/assembler/assembler_test.go	Updates the PrefetcherFactory mock callback signature to include Metrics.
docs/monitoring/metrics.md	Documents the three new Assembler latency metrics and fixes a minor formatting inconsistency.

Files not reviewed (1)

• node/assembler/mocks/prefetcher_factory.go: Generated file

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