Structured logging

[gloskull]

Motivation

• Provide OpenTelemetry-compatible structured logging alongside existing distributed tracing so on‑chain contracts can emit correlated logs and spans for off‑chain observability and dashboards.
• Expose a compact, contract-safe way to attach semantic‑convention attributes (service, component, critical‑path) to logs without adding storage writes on critical paths.

Description

• Add OpenTelemetry semantic names in tracing::semconv and a LogSeverity enum to represent severity text and numeric levels used by VeriNode.
• Introduce LogRecord and StructuredLogger with emit, info, and error helpers that publish structured log events under the log::record Soroban event topic and correlate with TraceId/SpanId.
• Reuse a shared encode_attributes helper and add critical_path_attributes to serialize attributes as key=value pairs for constrained contract callers.
• Refactor Tracer to reuse the attribute encoder, add end_span_at helper, and keep trace/span event format consistent with OpenTelemetry semantics.
• Update module docs in src/lib.rs to mention structured logging and add unit tests in src/tracing/test.rs plus Soroban test snapshots under test_snapshots/tracing to exercise emitted trace/log events.

Testing

• Ran cargo test tracing::test which executed the tracing/logging unit tests and snapshots with all tracing tests passing (12 passed).
• Ran the full test suite with cargo test; tracing tests passed but the overall run surfaced pre‑existing pool_manager unit test failures (full run: 134 passed; 17 failed) that are unrelated to the tracing/logging changes.
• Ran code formatting with cargo fmt / rustfmt during development.

Closes #74