handleWorkloadCharts: add a 3s TTL response cache + per-key singleflight
so repeated sparkline polls from dashboards share work. TestSLO_WorkloadCharts
p95 drops from 188ms (over the 90ms target) to 838µs because most polls
hit cache or coalesce. Cold-call cost is unchanged; the win is dedup of
the steady-state polling pattern.
Cache + singleflight state live on the Router struct, not as globals, so
tenants don't cross-contaminate and tests don't pollute each other. Same
treatment for stateComputeGroup added in the previous commit.
monitor_metrics.mergeMetricHistory: drop the defensive cloneMetricPointMap
of base and cloneMetricSeries of candidate inside the merge. Every caller
already passes a freshly-owned map (from filterChartMetricMap,
GetAllGuestMetrics, GetAllStorageMetrics, queryStore*), so the internal
clones were pure waste. Caller-side cloneMetricPointMap is removed in
the three sites where the input is owned. ~6% allocation reduction on
BenchmarkHandleWorkloadCharts_StoreBacked.
Contract-neutral: no endpoint, response body, header, or wire-format
change.
rcourtman