Add slow query log

[rdettai-sk]

Queries are now logged when they finish (success/error/cancelled). This makes it possible to extract slow queries quite easily.

This PR also cleans up an unused error field from the search response and improves the root search metrics with a too-many-split label.

Description

Describe the proposed changes made in this PR.

How was this PR tested?

Looking at the logs in the integration test (that's why I also modified the test log subscriber, the existing one has been anoying me for a while).

[Copilot]

Pull request overview

This PR aims to make it easier to extract slow queries by logging root search requests when they complete (success/error/partial), while also removing an unused errors field from the search response and refining root-search metrics with a “too-many-splits” label.

Changes:

• Add request-span based logging and status labeling for root search planning/execution, and enforce a max-splits-per-search limit via a dedicated TooManySplits error.
• Remove the deprecated/unused errors field from SearchResponse across protobuf, REST models, and tests/clients.
• Update test logging initialization in quickwit-common (moving from env_logger to tracing-subscriber under the testsuite feature).

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
quickwit/quickwit-serve/src/search_api/rest_handler.rs	Update REST handler tests to match removed errors field.
quickwit/quickwit-serve/src/jaeger_api/rest_handler.rs	Update Jaeger REST handler tests to match removed errors field.
quickwit/quickwit-search/src/service.rs	Remove errors initialization from SearchResponse.
quickwit/quickwit-search/src/search_response_rest.rs	Remove errors from REST response model and conversion.
quickwit/quickwit-search/src/root.rs	Add request span + split-limit enforcement; record request attributes for completion logs/metrics.
quickwit/quickwit-search/src/metrics_trackers.rs	Add completion logging/status labeling and request span propagation for root search futures.
quickwit/quickwit-search/src/error.rs	Introduce TooManySplits error variant + map to BadRequest.
quickwit/quickwit-rest-client/src/rest_client.rs	Update REST client tests to match removed errors field.
quickwit/quickwit-rest-client/src/models.rs	Remove errors from REST client model.
quickwit/quickwit-proto/src/codegen/quickwit/quickwit.search.rs	Remove errors from generated SearchResponse.
quickwit/quickwit-proto/protos/quickwit/search.proto	Reserve the deprecated errors field tag in SearchResponse.
quickwit/quickwit-jaeger/src/lib.rs	Update tests to match removed errors field.
quickwit/quickwit-integration-tests/src/tests/update_tests/mod.rs	Update integration test assertions now that errors field is gone.
quickwit/quickwit-common/src/lib.rs	Change test logging setup + gate it behind testsuite feature.
quickwit/quickwit-common/Cargo.toml	Remove env_logger; add optional tracing-subscriber for testsuite.
quickwit/Cargo.lock	Lockfile updates reflecting dependency changes.

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