feat(observability): add support for tracing

[thiagohsc21]

Summary

• Add APM tracing spans to all public evaluation methods (check_gate, get_dynamic_config, get_config, get_experiment, get_experiment_without_local_evaluation)
• Each span includes the gate/config/experiment name via otel.name for readable Datadog waterfalls
• A statsig.evaluation attribute distinguishes http (server round-trip) from local (in-memory) evaluations
• Differentiate get_experiment_remote span name from get_experiment to avoid trace collisions

Motivation

The statsig-rs SDK silently falls back to HTTP server-side evaluation when a ConditionType or OperatorType isn't implemented locally. These round-trips (~30-50ms) are invisible in APM, making it hard to diagnose unexpected latency in endpoints that appear to only do in-memory work.

What changes

• src/client.rs: wrap each evaluation method body with async { ... }.instrument(span).await
• No API changes, no new dependencies (uses tracing which is already a dependency)

What does NOT change

• No call site changes required by consumers
• No behavioral changes to evaluation logic
• log_event intentionally left uninstrumented (fire-and-forget analytics, not evaluation)

[EduardoGiacomini]

LGTM! Some points do fix:

1. Can you provide a minimal PR description write in Eglish?
2. It will be necessary change the crate version to publish the code.