docs(metrics): add README section, testData, and a scrape example

The metrics feature shipped in code only: no README section, no
metrics block in .traefik.yml testData, nothing under examples/. The
Go doc comments were the entire manual for a human operator or a
config-generating assistant.

Add a README Metrics section covering the served path, both auth
modes (admin bearer token, optional CIDR allowlist), the CIDR-sizing
warning with the live-reproduced SNAT failure mode, the modelLabel
cardinality trade-off, the exposed metric families, the cross-replica
aggregation contract, label escaping, and store-outage behaviour. Add
a MetricsConfig reference table matching the existing AdminConfig
one, and a metrics row in the top-level Config table.

Add metrics: {enabled: true} to .traefik.yml's testData, the
catalog-facing smoke-test config, deliberately without allowedCIDRs
so a copy-pasted example never skips the README's own sizing
guidance.

Add a metrics block to the Kubernetes Middleware example and a
VMServiceScrape resource: the target cluster runs VictoriaMetrics, not
Prometheus, so the CRD differs even though the exposition format does
not. Verified the VMServiceScrape shape (apiVersion, selector,
endpoints, path) against the victoriametrics/operator documentation
rather than assuming it. Notes that /metrics is served by Traefik
itself, not by any backend service, since this middleware intercepts
the path before a backend is ever reached.

.traefik.yml

@@ -28,3 +28,10 @@ testData:
       - name: tester
         group: default
         apiKey: test-user-key
+  # Prometheus text-exposition endpoint — see README.md#metrics. No
+  # allowedCIDRs here deliberately: this is the catalog-facing smoke-test
+  # config, and README.md#metrics has the full CIDR-sizing guidance
+  # (narrowest block only, never a whole RFC1918 range) that a
+  # copy-pasted example here would risk skipping past.
+  metrics:
+    enabled: true