caddyhttp: Collect metrics once per route instead of per handler (#7492)

* perf: collect metrics once per route instead of per handler (#4644)

Move Prometheus metrics instrumentation from the per-handler level to
the per-route level. Previously, every middleware handler in a route was
individually wrapped with metricsInstrumentedHandler, causing metrics to
be collected N times per request (once per handler in the chain). Since
all handlers in a route see the same request, these per-handler metrics
were redundant and added significant CPU overhead (73% of request
handling time per the original profiling).

The fix introduces metricsInstrumentedRoute which wraps the entire
compiled handler chain once in wrapRoute, collecting metrics only when
the route actually matches. The handler label uses the first handler's
module name, which is the most meaningful identifier for the route.

Benchmark results (5 handlers per route):
  Old (per-handler):  ~4650 ns/op, 4400 B/op, 45 allocs/op
  New (per-route):    ~940 ns/op,  816 B/op,   8 allocs/op
  Improvement:        ~5x faster, ~5.4x less memory, ~5.6x fewer allocs

Signed-off-by: Varun Chawla <varun_6april@hotmail.com>

* Remove unused metricsInstrumentedHandler code

Delete the metricsInstrumentedHandler type, its constructor, and
ServeHTTP method since they are no longer used after switching to
route-level metrics collection via metricsInstrumentedRoute. Also
remove the unused metrics parameter from wrapMiddleware and the
middlewareHandlerFunc test helper, and convert existing tests to
use the new route-level API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Address review feedback: restore comments, move function to bottom

- Move computeApproximateRequestSize back to bottom of file to minimize diff
- Restore all useful comments that were accidentally dropped
- Old metricsInstrumentedHandler already removed in previous commit

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Signed-off-by: Varun Chawla <varun_6april@hotmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>