The new test sends two paths one route answers and three requests no
route answers through the metrics middleware, and expects the handler
label to hold the route's pattern and "unmatched". It fails: the label
is each request's path, so every new path adds series.
Middleware gets a metricsRegistry field, nil being Prometheus' default
registry, so the test can register with a registry of its own: another
test in the package already registers with the default one.
Model: opus-5-5
