Convert telemetry code to use prometheus-cpp

src/EventHandler.h

@@ -9,7 +9,6 @@
 #include "zeek/Type.h"
 #include "zeek/ZeekArgs.h"
 #include "zeek/ZeekList.h"
-#include "zeek/telemetry/Counter.h"
 
 namespace zeek {
 
@@ -17,6 +16,10 @@ namespace run_state {
 extern double network_time;
 } // namespace run_state
 
+namespace telemetry {
+class IntCounter;
+}
+
 class Func;
 using FuncPtr = IntrusivePtr<Func>;
 
@@ -60,7 +63,8 @@ public:
     void SetGenerateAlways(bool arg_generate_always = true) { generate_always = arg_generate_always; }
     bool GenerateAlways() const { return generate_always; }
 
-    uint64_t CallCount() const { return call_count ? call_count->Value() : 0; }
+    // Returns the number of times this EventHandler has been called since startup.
+    uint64_t CallCount() const;
 
 private:
     void NewEvent(zeek::Args* vl); // Raise new_event() meta event.
@@ -74,7 +78,7 @@ private:
     bool generate_always;
 
     // Initialize this lazy, so we don't expose metrics for 0 values.
-    std::optional<zeek::telemetry::IntCounter> call_count;
+    std::shared_ptr<zeek::telemetry::IntCounter> call_count;
 
     std::unordered_set<std::string> auto_publish;
 };