fetch-router: Expose matched route on request context

[gyx1000]

This PR proposes a small fetch-router API addition that makes the matched route available from request context.

The motivation is future telemetry and observability work. I do not necessarily expect this exact PR to be accepted as-is; the goal is mostly to suggest a simple shape and ask whether Remix is already exploring telemetry support internally.

• Adds a MatchedRoute context key exported from @remix-run/fetch-router
• Stores the matched RouteEntry on the request context once a route and method have matched
• Makes the matched route readable from route middleware, route handlers, and router middleware after await next()

One telemetry use case is enriching an active OpenTelemetry HTTP span with the low-cardinality route pattern. For example, an instrumentation layer could report /products/:productId instead of /products/123, avoiding high-cardinality span names and attributes.

import { MatchedRoute } from '@remix-run/fetch-router'

async function telemetryMiddleware(context, next) {
  let response = await next()
  let matchedRoute = context.get(MatchedRoute)

  if (matchedRoute) {
    span.setAttribute('http.route', matchedRoute.pattern.source)
    span.updateName(`${context.method} ${matchedRoute.pattern.source}`)
  }

  return response
}

Is telemetry something the Remix team is already working on or planning for? If so, would exposing the matched route through request context fit that direction, or would you prefer a different extension point?