fix(monitoring): drop redundant relabels, resolve ServiceMonitor drift

Both new ServiceMonitors sat permanently OutOfSync. Cause: `action: replace`
is the API server default for relabeling rules, so the live object carried it
while the manifests omitted it.

Rather than write out the default to paper over drift, removed the rules that
were never needed:

- vllm.yaml: the namespace/pod/service relabelings were redundant —
  Kubernetes service discovery already supplies all three. Only `app` has to
  be derived (TEI panels aggregate by it and SD does not promote arbitrary
  pod labels). Kept that one, with an explicit action.
- api-gateway.yaml: the metricRelabelings block was a no-op — it mapped
  __address__ onto instance, which is already the default. Removed. The
  series inventory it was documenting moved up into the file header, which
  is where it belonged anyway.

k8s/infra/monitoring/servicemonitors/api-gateway.yaml

@@ -1,3 +1,25 @@
+# The gateway is the only source of per-request LLM latency covering EVERY
+# model, including the two Ollama ones (ornith:35b, qwen2.5:3b-instruct) that
+# expose no metrics endpoint at all. Series it emits, per
+# internal/observability/prometheus.go:
+#   llm_ttft_seconds                    histogram  time-to-first-token
+#   llm_itl_seconds                     histogram  inter-token latency
+#   llm_tokens_total                    counter    SSE chunks generated
+#   queue_full_requests_total           counter    backpressure rejects
+#   warmup_requests_delayed_total       counter    vLLM slot contention
+#   vllm_prefix_cache_hit_tokens_total  counter    prefix-cache hits
+#   request_duration_seconds            histogram  full request
+#
+# Caveat: these are derived from the SSE byte stream, not from the engine.
+# TTFT is request-start to first byte, ITL is the gap between SSE writes
+# carrying "data: ", and llm_tokens_total counts SSE chunks rather than true
+# tokens. They are also streaming-only — non-streaming requests record
+# nothing. Treat vLLM's own histograms as authoritative for `reasoning`.
+#
+# The Deployment already carried prometheus.io/scrape annotations, but
+# kube-prometheus-stack ignores pod annotations (no additionalScrapeConfigs,
+# serviceMonitorSelectorNilUsesHelmValues: false), so none of the above was
+# ever collected. This ServiceMonitor is what actually wires it up.
 apiVersion: monitoring.coreos.com/v1
 kind: ServiceMonitor
 metadata:
@@ -22,27 +44,3 @@ spec:
       path: /metrics
       scheme: http
 
-      # The gateway is the only source of per-request LLM latency that covers
-      # EVERY model, including the two Ollama ones that expose no metrics at
-      # all. Series it produces (internal/observability/prometheus.go):
-      #   llm_ttft_seconds              histogram  time-to-first-token
-      #   llm_itl_seconds               histogram  inter-token latency
-      #   llm_tokens_total              counter    tokens generated
-      #   queue_full_requests_total     counter    backpressure rejects
-      #   warmup_requests_delayed_total counter    vLLM slot contention
-      #   vllm_prefix_cache_hit_tokens_total counter prefix-cache hits
-      #   request_duration_seconds      histogram  full request
-      #
-      # The Deployment already carried prometheus.io/scrape annotations, but
-      # kube-prometheus-stack ignores pod annotations (no additionalScrape-
-      # Configs, serviceMonitorSelectorNilUsesHelmValues: false) — so none of
-      # the above was being collected. This ServiceMonitor is what actually
-      # wires it up.
-      metricRelabelings:
-        # 3 gateway replicas each build their own in-process histogram. Without
-        # this the label set is otherwise identical, so per-instance series
-        # would collide conceptually in sum()-by aggregations; instance is
-        # retained deliberately so per-replica skew stays visible.
-        - sourceLabels: [__address__]
-          targetLabel: instance
-          replacement: $1