Structured logging with open telemetry semantic conventions

[clintjeff2]

Motivation

• Provide a deterministic, schema'd contract event payload aligned with OpenTelemetry semantic conventions for critical billing paths.
• Make it easy for indexers and telemetry pipelines to correlate domain events with latency SLOs and error rates without increasing storage or cross-contract work.

Description

• Add OtelLogRecord contracttype and emit_otel_log helper to publish a single otel.log event with a 100 ms critical_path_budget_ms field in contracts/utility_contracts/src/lib.rs.
• Instrument critical billing paths to emit structured logs for meter.registered, meter.top_up, provider.claim, and usage.updated.
• Add unit tests validating the OtelLogRecord fields and that emit_otel_log publishes the otel.log event in contracts/utility_contracts/src/otel_logging_tests.rs.
• Document the design, OpenTelemetry field mapping, monitoring/alerting guidance, and blue-green/canary deployment runbook in contracts/docs/specs/otel-structured-logging.md.

Testing

• Ran rustfmt on the new test file successfully.
• Attempted cargo test --manifest-path contracts/utility_contracts/Cargo.toml otel_logging_tests --lib, but execution was blocked by pre-existing, unrelated compile errors in the utility_contracts crate so the new tests could not be fully validated.
• Ran cargo check --manifest-path contracts/utility_contracts/Cargo.toml --lib, which also surfaced unrelated compile failures in the crate (duplicate constant, symbol length issues, and several pre-existing API/signature mismatches).

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Closes #73