Fix issue 2098: Add opt-in structured logging, metrics, and tracing

[lqminhhh]

feat(observability): add opt-in structured logging, metrics, and tracing

Description

This PR adds an opt-in observability foundation for Qlib.

It introduces structured JSON logging, lightweight metrics collection, metrics export/reporting, and basic trace/span support while preserving Qlib's existing default logging behavior.

Main changes:

• Added JSONFormatter for structured JSON logs.
• Added compact structured logging config support:

qlib.init(logging_config={"structured": True, "format": "json"})

• Added lightweight metrics support:

qlib.init(metrics_config={"enabled": True})

• Added counters, gauges, timings, timer context manager, export, and summary reporting.
• Added cache instrumentation for dataset/calendar cache hit, miss, and load duration metrics.
• Added lightweight tracing support:

qlib.init(tracing_config={"enabled": True})

• Added trace_span(...) context manager.
• Added trace fields to structured logs when tracing is enabled.
• Added trace spans around cache dataset/calendar operations.

Motivation and Context

Related to #2098.

Qlib currently has plain-text logging and some isolated timing/experiment metrics, but it does not have a unified opt-in observability layer for structured logs, runtime metrics, and trace context.

This PR keeps observability disabled by default and does not introduce mandatory external monitoring dependencies such as Prometheus or OpenTelemetry.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Focused tests run:

python -m pytest tests/test_log.py tests/test_metrics.py tests/test_trace.py -q

Result:

24 passed

Also manually tested with a real-data Qlib notebook using D.features(...) to verify structured logs, metrics export/summary, and trace spans.

Screenshots of Test Results (if appropriate)

1. Pipeline test: Not run.
2. Your own tests:

24 passed

Types of changes

• Fix bugs
• Add new feature
• Update documentation

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