docs: Unknown routes are now recorded in HTTP metrics (path="other"), access logs, and request IDs

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What this PR does

Are requests to unknown paths visible in SMG's HTTP metrics, access logs, and request-ID correlation, and under what path label?

Why we need it

Source change: smg-project/smg@0a58a6c

Commit 0a58a6c (#2679, landed after the v1.11.0 version bump) added attach_edge_layers in model_gateway/src/server.rs: the sink_handler 404 fallback is now registered before the body-limit, logging (middleware::create_logging_layer), HTTP-metrics (HttpMetricsLayer), RequestIdLayer, and CORS layers, with the test unknown_routes_cross_the_edge_middleware asserting an unknown route gets x-request-id. In current source, model_gateway/src/middleware/metrics.rs matched_path_label maps requests with no MatchedPath to "other" (test unmatched_path_collapses_to_other), and model_gateway/src/middleware/logging.rs RequestLogger/ResponseLogger record smg_http_requests_total and log started/finished lines inside the http_request span for every request crossing the layer. Three current docs claims are now wrong: src/lib/content/reference/metrics.md line 51 says requests that match no route "get a bare 404 from a fallback outside the metrics layer and are not recorded"; src/lib/content/getting-started/logging.md line 142 says "A request to an unknown path gets a bare 404 and is not logged" (and lines 132/465 scope the http_request span and request IDs to requests "that match a route"); src/lib/content/getting-started/monitoring.md line 76 says the http_request span is exported "for each HTTP request that matches a route". This is one independent concern: a single fallback relocation makes unknown routes observable across metrics, logs, and request IDs. The fix is on main only (post-v1.11.0), so the edit should describe main and may note that v1.11.0 and earlier skipped these requests.

Scope: observability / unknown-route-edge-middleware. Other concerns are deferred.

How to test

• Passed the documentation path and size guard (under 1000 added plus deleted lines; no file-count limit).
• Passed an independent accuracy and single-concern review.
• Passed git diff --check and pnpm check and the production pnpm build.

Source-level follow-up validation

The outer CORS layer handles OPTIONS directly; those requests bypass the inner logging, metrics, and request-ID layers. The documentation explicitly retains this exception. The corrected diff passed git diff --check, pnpm check, and pnpm build.

Checklist

• Tests added/updated (if applicable)
• Docs updated (if applicable)
• Commit signed off by XinyueZhang369 zoeyzhang369@gmail.com

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