http: set r.Pattern in ServeHTTP before middleware dispatch

[raphael]

Summary

Follow-up to #3897. The previous change set r.Pattern inside the handler wrapper, which runs after otelhttp.RequestTraceAttrs reads it. This meant the http.route span attribute was never populated — only the span name was updated.

Root cause

otelhttp reads r.Pattern in RequestTraceAttrs to set http.route at span-start time (before calling next.ServeHTTP). Setting r.Pattern inside the handler is too late for this attribute.

Fix

Override ServeHTTP on the mux to pre-resolve the route via chi.Match and set r.Pattern before chi's middleware chain executes. This ensures that:

• Middlewares registered via mux.Use() (e.g., otelhttp.NewMiddleware) see r.Pattern at span-start time
• http.route is set as a span attribute (not just the span name)
• Metrics also get the route via r.Pattern

The recommended pattern becomes:

mux := goahttp.NewMuxer()
mux.Use(otelhttp.NewMiddleware("service"))

The per-handler r.Pattern setting from #3897 is kept as well — it ensures r.Pattern is correct inside the handler itself.

Test plan

• New TestRequestPatternInMiddleware — verifies r.Pattern is visible in Use() middlewares (simulating otelhttp)
• Existing TestRequestPattern — handler-level access still works
• All mux tests pass