middleware: set http.Request.Pattern to the matched route

[delthas]

Motivation

Since Go 1.22, http.Request.Pattern carries the route pattern that matched a request. Router-agnostic observability middleware — OpenTelemetry's otelhttp, Prometheus, structured logging — reads it to label spans/metrics by route template (bounded cardinality) rather than raw path. net/http.ServeMux populates it; third-party routers increasingly do the same.

go-openapi already matches the route (middleware.MatchedRouteFrom), but consumers must hand-write a bespoke middleware to copy the matched pattern into their tracing/metrics labels — see e.g. go-swagger/go-swagger#3124, where users assemble an otelhttp wrapper by hand. Populating Request.Pattern lets standard middleware pick up the route with zero go-openapi-specific code (e.g. otelhttp.NewHandler then names spans by the matched route).

Change

In Context.RouteInfo, when a route matches, set request.Pattern to BasePath + PathPattern (e.g. /api/pets/{id}). It is set in place — mirroring net/http.ServeMux — so a handler wrapping the router observes it (a WithContext copy would only be visible downstream). It stays empty for unmatched requests, exactly like ServeMux. No behavior change for code that does not read Request.Pattern.

Request.Pattern requires Go 1.22+; this module is already on go 1.25.

Test

Added TestRouteInfoSetsRequestPattern (petstore fixture): asserts Request.Pattern equals the matched BasePath+PathPattern on both the returned and the in-place request, and stays empty when no route matches.

[fredbi]

Thank you @delthas I am going to review this ASAP (most likely tomorrow). See ya.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.36%. Comparing base (fd26cf9) to head (718c0f6).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   83.42%   83.36%   -0.07%     
==========================================
  Files          64       64              
  Lines        4520     4521       +1     
==========================================
- Hits         3771     3769       -2     
- Misses        581      583       +2     
- Partials      168      169       +1     

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[fredbi]

Looks good to me