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/multiChanTicker.go

@@ -8,6 +8,8 @@
 package multiChanTicker
 
 import (
+	"fmt"
+	"sync"
 	"time"
 
 	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
@@ -15,6 +17,7 @@ import (
 
 type multiChanTicker struct {
 	ticker   *time.Ticker
+	mutex    sync.Mutex // protects channels, which is appended to while the tick goroutine iterates it
 	channels []chan time.Time
 	done     chan bool
 }
@@ -40,21 +43,30 @@ func (t *multiChanTicker) Init(duration time.Duration) {
 				return
 			case ts := <-t.ticker.C:
 				cclog.ComponentDebug("MultiChanTicker", "Tick", ts)
-				for _, c := range t.channels {
+				t.mutex.Lock()
+				for i, c := range t.channels {
+					// Non-blocking send: a consumer that has not yet read the
+					// previous tick must not stall the ticker, otherwise
+					// time.Ticker silently drops fires for ALL consumers
 					select {
-					case <-t.done:
-						done()
-						return
 					case c <- ts:
+					default:
+						cclog.ComponentWarn("MultiChanTicker", fmt.Sprintf("consumer %d did not read previous tick, dropping tick %v", i, ts))
 					}
 				}
+				t.mutex.Unlock()
 			}
 		}
 	}()
 }
 
 func (t *multiChanTicker) AddChannel(channel chan time.Time) {
+	if cap(channel) == 0 {
+		cclog.ComponentWarn("MultiChanTicker", "unbuffered channel registered, ticks may be dropped if the consumer is not ready")
+	}
+	t.mutex.Lock()
 	t.channels = append(t.channels, channel)
+	t.mutex.Unlock()
 }
 
 func (t *multiChanTicker) Close() {