[CT-40] Add consistent metrics instrumentation across all handlers

[mftee]

Overview

MetricsRegistry (increment_request_count, increment_cache_hits/misses, increment_error_count) is actively used inside verify_document and batch_verify_documents, but submit_document, revoke_document, and record_transfer never call increment_request_count(), and record_transfer only increments error_count on failure paths, never request_count on the happy path. /metrics currently undercounts real traffic.

File

contract/src/lib.rs (submit_document, revoke_document, record_transfer, and CT-31/CT-32's implementations)

Acceptance Criteria

• Every handler calls metrics.increment_request_count() at the start of its logic
• Every handler's error paths call metrics.increment_error_count()
• Consider adding a per-route counter label (or a new metric) so /metrics can distinguish verify vs submit vs revoke vs transfer volume, not just a single global counter
• Add a test asserting the metrics output includes nonzero counts after exercising each endpoint