telemetry: Invoke Telemetry::sync() only at scrape/collection time

This stops invoking Telemetry::sync() via a scheduled event and instead
only invokes it on-demand. This makes metric collection network time
independent and lazier, too.

With Prometheus scrape requests being processed on Zeek's main thread
now, we can safely invoke the script layer Telemetry::sync() hook.

Closes #3947

scripts/base/init-bare.zeek

@@ -5933,6 +5933,21 @@ export {
 		sum: double;
 	};
 
+	## Telemetry sync hook.
+	##
+	## This hook is invoked when metrics are requested via functions
+	## :zeek:see:`Telemetry::collect_metrics` and :zeek:see:`Telemetry::collect_histogram_metrics`,
+	## or just before Zeek collects metrics when being scraped through
+	## its Prometheus endpoint.
+	## Script writers can use it to synchronize (or mirror) metrics with the
+	## telemetry subsystem. For example, when tracking table or value
+	## footprints with gauges, the value in question can be set on an actual
+	## :zeek:see:`Telemetry::Gauge` instance during execution of this hook.
+	##
+	## Implementations should be lightweight, this hook may be called
+	## multiple times per minute.
+	global sync: hook();
+
 	type MetricVector : vector of Metric;
 	type HistogramMetricVector : vector of HistogramMetric;