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/scheduler.py

@@ -64,11 +64,13 @@ async def _do_scrape(app: "Quart") -> None:
     from .scraper import (
         Baselines,
         api_base,
+        combine,
         compute_global_stats,
         compute_router_metrics,
         compute_status_codes,
-        extract_certs,
-        fetch_metrics,
+        fetch_instances,
+        merge_global_stats,
+        unique_certs,
         fetch_error_page_services,
         fetch_router_domains,
     )
@@ -77,7 +79,7 @@ async def _do_scrape(app: "Quart") -> None:
     metrics_url: str = app.config["PLUGINS"]["traefik"]["metrics_url"]
 
     try:
-        current = await fetch_metrics(metrics_url)
+        scrapes = await fetch_instances(metrics_url)
     except Exception:
         logger.exception("Traefik scrape failed (url=%s)", metrics_url)
         return
@@ -95,11 +97,12 @@ async def _do_scrape(app: "Quart") -> None:
 
     if _baselines is None:
         _baselines = Baselines()
-    previous, elapsed = _baselines.swap(current, time.monotonic())
+    now = time.monotonic()
+    current, previous, elapsed = combine([(scrape, *_baselines.swap(scrape, now)) for scrape in scrapes])
     router_metrics = compute_router_metrics(current, previous, elapsed)
-    global_stats = compute_global_stats(current, previous)
+    global_stats = merge_global_stats([compute_global_stats(scrape, None) for scrape in scrapes])
     status_codes = compute_status_codes(current, previous)
-    cert_list = extract_certs(current)
+    cert_list = unique_certs(scrapes)
 
     scraped_at = datetime.now(timezone.utc)