fix(traefik): latency is each scrape's own percentile, every label set summed (#5755)

#5755 "Traefik latency was a percentile of every response since Traefik
started, over unsummed label sets". _percentile_ms read the duration
histogram as it stood, which counts from Traefik's start, so p50/p95/p99 were
lifetime figures that barely moved. It also walked every code/method label
set's buckets as one unsummed list, so equal counts from two sets returned
an edge exactly: nextcloud's flat 100.0 ms p95.

service_buckets() now takes each bucket's rise since the previous scrape,
summed per le over every label set. A scrape with no responses, and the
first scrape, record no latency: traefik_007_latency_nullable lets the
columns be NULL, the batch skips the figure, and the page shows "—".

Found while preparing #5741 "Traefik routers are graded against their own
normal", which grades p95 against a learned normal.

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

plugins/traefik/models.py

@@ -18,9 +18,10 @@ class TraefikMetric(Base):
     request_rate: Mapped[float] = mapped_column(Float, nullable=False)
     error_rate_4xx_pct: Mapped[float] = mapped_column(Float, nullable=False)
     error_rate_5xx_pct: Mapped[float] = mapped_column(Float, nullable=False)
-    latency_p50_ms: Mapped[float] = mapped_column(Float, nullable=False)
-    latency_p95_ms: Mapped[float] = mapped_column(Float, nullable=False)
-    latency_p99_ms: Mapped[float] = mapped_column(Float, nullable=False)
+    # None for a scrape with no responses: an idle router has no latency (#5741).
+    latency_p50_ms: Mapped[float | None] = mapped_column(Float, nullable=True)
+    latency_p95_ms: Mapped[float | None] = mapped_column(Float, nullable=True)
+    latency_p99_ms: Mapped[float | None] = mapped_column(Float, nullable=True)
     response_bytes_rate: Mapped[float] = mapped_column(Float, nullable=False, default=0.0)