* n8n: add container-based config discovery support

* n8n: pass explicit config to dd_agent_check in regular e2e test

Without an explicit config, dd_agent_check relies on conf.d which now
includes the mounted auto_conf.yaml (from get_e2e_discovery_metadata).
This causes autodiscovery to fire an extra check instance alongside the
two static ones, corrupting metric assertions in test_check_n8n_e2e.

Passing the instances explicitly forces --config-file, which overrides
conf.d and isolates the test from autodiscovery — matching the krakend
reference pattern.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Revert "n8n: pass explicit config to dd_agent_check in regular e2e test"

This reverts commit 7f26c0f.

* n8n: add discovery stub files

* n8n: remove ad_identifiers from discovery spec stanza

* n8n: update discovery.py to new generated format

* n8n: add test_e2e_discovery_all_candidates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* n8n: fix import sort order in test_e2e.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: restore blank line between import groups in test_e2e.py

* Add auto_conf.yaml section to spec.yaml so it is generated from spec.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Regenerate auto_conf.yaml with doc comments from spec template.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Use discovery/openmetrics_from_ports template in n8n spec, consistent with krakend

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Assert both main and worker instances are discovered in test_e2e_discovery.

Both containers share the same image and are both discovered: main on port 5678
(via the hint), worker on port 5680 (via the port fallback). Together they cover
the full metric set so symmetric inclusion can be enabled. Uses
discovery_min_instances=2 as suggested.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Skip test_e2e_discovery in lab mode as auto_conf.yaml is not mounted.

Follows the same approach as krakend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Extract common assertions into helper to align test_e2e_discovery with test_check_n8n_e2e.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Restore worker readiness comment and explain missing n8n_process tags in discovery test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>