feat(traefik): scrape every Traefik replica behind the metrics URL and add them up (#5844)

Through one address Steward read whichever replica answered, and with
host-mode ports behind a keepalived VIP that can be the replica serving
nothing: since the 18:43 restart every scrape landed on Docker03's idle
Traefik, which has no traefik_service_* series, so no router was stored.

A metrics_url host name that resolves to several addresses (a swarm
service's tasks.<service>, a Kubernetes headless Service, round-robin DNS)
is now scraped address by address, with the original Host header and TLS
name. Answers from one process are kept once; a replica that doesn't answer
is skipped with a warning. Requests, bytes and latency buckets are summed
over the replicas that have a baseline, so the sum's rise is the sum of each
one's own rise; certificates are listed once; Traefik's own figures add
connections and memory and keep the newest reload. A name with one address
is one scrape, as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/traefik/plugin.yaml

@@ -13,6 +13,9 @@ tags:
   - access-log
 
 config:
+  # A host name that resolves to several Traefiks (a swarm service's
+  # tasks.<service>, a Kubernetes headless Service) is scraped address by
+  # address and added up, so every replica's traffic counts.
   metrics_url: "http://localhost:8080/metrics"
   # Traefik's API, for which hosts each router serves (so a certificate's
   # expiry shows on its router). Empty: /api on the metrics URL's host.