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>

docs/configuration.md

@@ -19,6 +19,8 @@ The global file contains the paths to the other four files and some global optio
 
 Be aware that the paths are relative to the execution folder of the cc-metric-collector binary, so it is recommended to use absolute paths.
 
+The optional `channel_buffer_size` option sets the capacity of the internal channels between the components (collectors → router → sinks). If unset, it defaults to `max(200, 24 * number of CPUs)` so that one interval's burst of per-hwthread metrics fits without back-pressuring the collectors on nodes with many cores.
+
 ## Component configuration
 
 The others are mainly list of of subcomponents: the collectors, the receivers, the router and the sinks. Their role is best shown in a picture: