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/templates/traefik/rows.html

@@ -34,6 +34,10 @@
 {% endif %}
 {% endmacro %}
 
+{# A router that has answered nothing since Steward started has no latency yet. #}
+{% macro ms(v) %}{% if v is none %}—{% else %}{{ "%.0f"|format(v) }} ms{% endif %}{% endmacro %}
+{% macro latency_cell(v) %}<td{% if v is not none %} class="tone-{{ status_info(status_class(v, 'latency'))[0] }}"{% endif %}>{{ ms(v) }}</td>{% endmacro %}
+
 {% macro router_card(r) %}
 <div class="card">
     <div class="host-health-head">
@@ -54,8 +58,8 @@
     <table class="table" style="font-size:0.85rem;">
         <tr><td class="field-hint">Req/s</td><td>{{ "%.2f"|format(r.latest.request_rate) }}</td><td>{{ r.sparkline_req|safe }}</td></tr>
         <tr><td class="field-hint">Bandwidth</td><td>{{ r.latest.response_bytes_rate|rate }}</td><td>{{ r.sparkline_bytes|safe }}</td></tr>
-        <tr><td class="field-hint">p95</td><td class="tone-{{ status_info(status_class(r.latest.latency_p95_ms, 'latency'))[0] }}">{{ "%.0f"|format(r.latest.latency_p95_ms) }} ms</td><td>{{ r.sparkline_p95|safe }}</td></tr>
-        <tr><td class="field-hint">p99</td><td>{{ "%.0f"|format(r.latest.latency_p99_ms) }} ms</td><td></td></tr>
+        <tr><td class="field-hint">p95</td>{{ latency_cell(r.latest.latency_p95_ms) }}<td>{{ r.sparkline_p95|safe }}</td></tr>
+        <tr><td class="field-hint">p99</td><td>{{ ms(r.latest.latency_p99_ms) }}</td><td></td></tr>
         <tr><td class="field-hint">4xx</td><td class="{{ 'tone-warn' if r.latest.error_rate_4xx_pct > error_4xx else '' }}">{{ "%.1f"|format(r.latest.error_rate_4xx_pct) }}%</td><td>{{ r.sparkline_4xx|safe }}</td></tr>
         <tr><td class="field-hint">5xx</td><td class="{{ 'tone-crit' if r.latest.error_rate_5xx_pct > error_5xx else '' }}">{{ "%.1f"|format(r.latest.error_rate_5xx_pct) }}%</td><td>{{ r.sparkline_5xx|safe }}</td></tr>
     </table>
@@ -88,7 +92,7 @@
       <td style="word-break:break-all;">{{ r.name }}</td>
       <td>{{ "%.2f"|format(r.latest.request_rate) }}</td>
       <td>{{ r.latest.response_bytes_rate|rate }}</td>
-      <td class="tone-{{ status_info(status_class(r.latest.latency_p95_ms, 'latency'))[0] }}">{{ "%.0f"|format(r.latest.latency_p95_ms) }} ms</td>
+      {{ latency_cell(r.latest.latency_p95_ms) }}
       <td>{{ "%.1f"|format(r.latest.error_rate_4xx_pct) }}%</td>
       <td>{{ "%.1f"|format(r.latest.error_rate_5xx_pct) }}%</td>
     </tr>