Fix metric timeseries gaps on nodes with many cores

On nodes with >300 cores, one interval's burst of per-hwthread metrics
overran the fixed 200-slot channels. With blocking sends at every hop,
sink back-pressure propagated to the collectors, the collection round
exceeded the interval, and time.Ticker silently dropped the missed
ticks - whole intervals were skipped without any log message.

- multiChanTicker: deliver ticks non-blockingly and warn when a
  consumer misses a tick instead of stalling all consumers; guard the
  channel list with a mutex (data race with AddChannel)
- collectorManager: run the collection round detached from the tick
  loop, skip-and-warn when a round is still running, log per-collector
  and per-round durations at debug level, close serial collectors on
  shutdown
- metricRouter: buffer the interval timestamp channel and drain it
  before stamping, so metrics never carry the previous interval's
  timestamp; warn when the collector input channel is full at tick time
- main: scale the inter-manager channels to max(200, 24*NumCPU),
  overridable with the new optional channel_buffer_size config option
- add first unit tests for ticker, collector manager and router

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/multiChanTicker/README.md

@@ -30,8 +30,8 @@ Afterwards, you can add channels:
 
 ```golang
 t := MultiChanTicker(duration)
-c1 := make(chan time.Time)
-c2 := make(chan time.Time)
+c1 := make(chan time.Time, 1)
+c2 := make(chan time.Time, 1)
 t.AddChannel(c1)
 t.AddChannel(c2)
 
@@ -46,3 +46,5 @@ for {
 ```
 
 The result should be the same `time.Time` output in both channels, notified "simultaneously".
+
+Ticks are delivered with a non-blocking send: a consumer that has not yet read the previous tick does not stall the ticker (which would silently drop `time.Ticker` fires for all consumers); instead, the tick for that consumer is skipped and a warning is logged. Register buffered channels (capacity 1) so a consumer that is briefly busy at tick time does not lose the tick.