Add devnet-0 metrics

[KatyaRyazantseva]

This PR adds Devnet-0 metrics.

• Added Counter metric type with and without labels.
• Added Histogram metric type with and without labels, with configurable buckets.
• Implemented HistogramTimer helper for automatic timing measurements
• Integrated Devnet-0 metrics.

[Copilot]

Pull Request Overview

This PR adds comprehensive metrics support for Devnet-0, introducing Counter and Histogram metric types alongside the existing Gauge metrics. The changes enable performance monitoring and operational visibility across state transitions, attestation processing, and validator management.

Key changes:

• Implemented Counter and Histogram metric types with optional label support and configurable buckets
• Added HistogramTimer helper class for automatic timing measurements with RAII pattern
• Integrated metrics throughout blockchain components (STF, fork choice, validator registry)

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/metrics/metrics.hpp	Added Counter/Histogram interfaces and HistogramTimer helper class
src/metrics/impl/metrics_impl.hpp	Extended MetricsImpl with Counter and Histogram method declarations
src/metrics/impl/metrics_impl.cpp	Implemented Counter and Histogram metric registration and accessors
src/blockchain/state_transition_function.hpp	Added metrics dependency to STF class
src/blockchain/state_transition_function.cpp	Integrated timing and counting metrics for state transitions, slots, blocks, and attestations
src/blockchain/impl/validator_registry_impl.hpp	Added metrics parameter to ValidatorRegistryImpl constructors
src/blockchain/impl/validator_registry_impl.cpp	Implemented validator count metric
src/blockchain/fork_choice.cpp	Added attestation validation and block processing metrics
src/blockchain/state_transition_metrics.def	Defined STF-related metrics (justification, finalization, timing counters)
src/blockchain/impl/validator_metrics.def	Defined validator count gauge metric
src/blockchain/fork_choice_metrics.def	Updated with attestation validation and block processing metrics
src/app/application_metrics.def	Updated documentation format for consistency
src/metrics/all_metrics.def	Added includes for new metric definition files
tests/mock/blockchain/metrics_mock.hpp	Extended mock with Counter and Histogram implementations
tests/unit/blockchain/state_transition_function_test.cpp	Updated test to pass metrics mock to STF
tests/unit/utils/validator_registry_test.cpp	Updated tests to pass metrics mock to ValidatorRegistryImpl

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