refactor(middleware): migrate PrometheusMiddleware to ASGIMiddleware

[Kumzy]

Part of #4009. Stacked PR to avoid conflicts in the changelog and whats-new docs.

Moving PrometheusMiddleware from AbstractMiddleware to ASGIMiddleware.

The constructor takes keyword arguments now instead of an app and a PrometheusConfig object, with each argument defaulting to the matching config field. PrometheusConfig itself is unchanged and stays the way to configure this: its middleware property now returns a configured PrometheusMiddleware instance instead of a DefineMiddleware, so middleware=[config.middleware] keeps working as before. The excluded_http_methods check stays inside handle().

Same breaking change as the other PRs in the series: exclude patterns now match the handler's path template at startup instead of the request path (documented in the changelog and whats-new-3).

Same subtlety as allowed-hosts: PrometheusConfig.scopes is user-configurable and mounted ASGI apps receive both http and websocket connections at runtime, so configured scopes are also enforced per connection via should_bypass_for_scope. Without this, restricting to scopes={"websocket"} would have silently kept instrumenting http connections through ASGI mounts.

Added some tests, covering the prometheus-specific behavior of this migration:

• One that checks mounted ASGI apps are still instrumented. Dropping ASGI from the scopes tuple was undetected before.
• One that checks configured scopes hold for connections through mounts (http through a mount is NOT recorded with scopes={"websocket"}). It was failing.
• One that checks group_path=False uses the request path instead of the template. The False branch was not covered before.
• One that checks a subclass set via middleware_class works with the new handle contract, since this PR changes that hook (no more self._config).
• Extended the buckets assertion so default bucket boundaries must be absent - without this, dropping buckets from the property was undetected.

Two pre-existing quirks found while porting, unchanged here (possible follow-ups): request.method raises KeyError for websocket connections when excluded_http_methods is set, and PrometheusConfig.exclude_unhandled_paths is a dead field nothing reads.

---

📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/5006

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.21%. Comparing base (84420b9) to head (ceecd23).

Additional details and impacted files

@@                Coverage Diff                @@
##           mw/rate-limit    #5006      +/-   ##
=================================================
+ Coverage          67.19%   67.21%   +0.02%     
=================================================
  Files                293      293              
  Lines              15404    15415      +11     
  Branches            1750     1751       +1     
=================================================
+ Hits               10350    10361      +11     
  Misses              4900     4900              
  Partials             154      154              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.