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>

internal/metricRouter/metricCache.go

@@ -79,7 +79,7 @@ func (c *metricCache) Init(output chan lp.CCMessage, ticker mct.MultiChanTicker,
 
 // Start starts the metric cache
 func (c *metricCache) Start() {
-	c.tickchan = make(chan time.Time)
+	c.tickchan = make(chan time.Time, 1)
 	c.ticker.AddChannel(c.tickchan)
 	// Router cache is done
 	done := func() {