observability: name the prefix-cache series that actually exist

vllm:prefix_cache_hit_rate does not exist in this build; the exported pair is
vllm:prefix_cache_hits_total / vllm:prefix_cache_queries_total, whose rates give
the hit rate. A comment naming a series nobody can query is worse than no
comment.

shared/prometheus/prometheus.yml

@@ -57,9 +57,11 @@ scrape_configs:
   # LLM fleet. Scraped on the LAN addresses, not the guarded vhosts: these are
   # machine clients and Prometheus cannot solve an Anubis challenge.
   # frank-medium is vLLM, and its series are the ones that explain behaviour
-  # under load — vllm:kv_cache_usage_perc (how full the pool is),
-  # vllm:prefix_cache_hit_rate (whether pinned conversations are being reused),
-  # vllm:num_requests_waiting (queue depth), and the request-latency histograms.
+  # under load — vllm:kv_cache_usage_perc (how full the per-card pool is),
+  # vllm:num_requests_waiting (queue depth), the request-latency histograms,
+  # and the prefix-cache pair vllm:prefix_cache_hits_total over
+  # vllm:prefix_cache_queries_total (their rates give the hit rate, which says
+  # whether pinned conversations are actually being reused).
   # frank-large is llama.cpp and exposes throughput counters under llamacpp:.
   - job_name: "frank-medium"
     metrics_path: "/metrics"