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_test.go

@@ -0,0 +1,51 @@
+package multiChanTicker
+
+import (
+	"testing"
+	"time"
+)
+
+// A consumer that never reads its channel must not stall the ticker
+// or starve the other consumers
+func TestStalledConsumerDoesNotStarveOthers(t *testing.T) {
+	stalled := make(chan time.Time, 1) // never read
+	fast := make(chan time.Time, 1)
+
+	ticker := NewTicker(10 * time.Millisecond)
+	defer ticker.Close()
+	ticker.AddChannel(stalled)
+	ticker.AddChannel(fast)
+
+	received := 0
+	deadline := time.After(5 * time.Second)
+	for received < 5 {
+		select {
+		case <-fast:
+			received++
+		case <-deadline:
+			t.Fatalf("received only %d ticks while another consumer stalled", received)
+		}
+	}
+}
+
+// Close() must return promptly even if no consumer reads its channel
+func TestCloseWithStalledConsumer(t *testing.T) {
+	stalled := make(chan time.Time, 1) // never read
+
+	ticker := NewTicker(10 * time.Millisecond)
+	ticker.AddChannel(stalled)
+
+	// Let some ticks fire and be dropped
+	time.Sleep(50 * time.Millisecond)
+
+	closed := make(chan struct{})
+	go func() {
+		ticker.Close()
+		close(closed)
+	}()
+	select {
+	case <-closed:
+	case <-time.After(5 * time.Second):
+		t.Fatal("Close() blocked with a stalled consumer")
+	}
+}