Add Phase 0 trace query spans

[thorrester]

Pull Request

Short Summary

Adds Phase 0 trace-query observability spans for the in-scope Delta-backed trace handlers and query paths.

Context

This PR depends on the Phase 0 observability contract merged in #297. It instruments the five assigned trace handlers only:

• paginated_traces
• get_trace_spans
• trace_metrics
• get_trace_spans_by_id
• v1_otel_traces

The change adds documented #[tracing::instrument] fields for trace query metadata, plus nested spans for table resolution, logical planning, physical planning, collection, Arrow conversion, trace tree build, and response serialization where those stages are available in the assigned scope.

The Delta trace engine now emits lifecycle spans for table load, snapshot refresh, catalog swap, optimize, and incremental update paths inside the assigned engine.rs scope.

scouter_refresh_on_request_path_total was not emitted because the assigned files do not expose a RefreshOrigin::Request-style origin signal. I did not add a broader counter without that exact origin.

The Phase 0 contract constants exist under scouter_tracing::tracer::phase0_observability, but the in-scope crates do not depend on scouter-tracing, and Cargo manifests were outside this task's write scope. To keep the implementation scoped, the contract names are mirrored locally in the touched modules.

Verification run from /tmp/scouter-phase0-query-delta-spans:

• make lints passed.
• cargo test -p scouter-dataframe --all-features -- --nocapture --test-threads=1 passed after removing generated local Delta test storage from a previous interrupted run.
• cargo test -p scouter-server --all-features trace -- --nocapture --test-threads=1 passed.

Python verification was skipped because this change does not alter PyO3-exposed types or Python behavior.

Is this a Breaking Change?

No. This adds observability instrumentation only and does not change public API contracts, storage schemas, or Python behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (olap-serving-layer@7b19ab3). Learn more about missing BASE report.

Additional details and impacted files

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