Attach lock context to overloaded exceptions for better metrics

[dpep]

Motivation

• Make it possible for middleware to report lock-based metrics (capacity/contention) even when a limiter raises an overloaded error by carrying lock context on the exception.
• Improve observability of overload events so StatsD and tracing middleware can record meaningful stats instead of only noting an error.

Description

• Add Berater::Overloaded#lock and an initializer that accepts a Lock instance so overload exceptions can carry lock details.
• Update concrete limiters (StaticLimiter, RateLimiter, ConcurrencyLimiter, Inhibitor) to raise Overloaded.new(Lock.new(...)) with the appropriate capacity and count/contention when the request is refused.
• Update Statsd middleware to use error.lock when no lock was returned so lock gauges are emitted for overload events.
• Update Trace middleware to retrieve lock info from error.lock when applicable so traces can still tag contention on overloads, and update specs to assert the new behavior.

Testing

• Updated middleware specs spec/middleware/statsd_spec.rb and spec/middleware/trace_spec.rb to validate lock-based metrics/tags are captured from the overloaded exception's lock; these specs were modified alongside code changes.
• Attempted to run relevant specs with bundle exec rspec spec/middleware/statsd_spec.rb spec/middleware/trace_spec.rb spec/concurrency_limiter_spec.rb spec/rate_limiter_spec.rb spec/static_limiter_spec.rb spec/inhibitor_spec.rb but the environment cannot run them here: bundler: command not found: rspec.

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Codex Task