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

src/telemetry/Manager.h

@@ -259,6 +259,13 @@ private:
     RecordValPtr GetMetricOptsRecord(const prometheus::MetricFamily& metric_family);
     void BuildClusterJson();
 
+    /**
+     * Runs the Telemetry::sync() hook in Zeek script land.
+     */
+    void InvokeTelemetrySyncHook();
+
+    bool in_sync_hook = false;
+
     std::map<std::string, std::shared_ptr<MetricFamily>> families;
     std::map<std::string, RecordValPtr> opts_records;