new(userspace/engine): create stats_manager inside falco engine

This is a porting of what we had inside the Lua codebase. This now handles the single responsibility
of gathering stats about rule-event matching, and of formatting them to print them to the user.

Signed-off-by: Jason Dellaluce <jasondellaluce@gmail.com>

userspace/engine/stats_manager.h

@@ -0,0 +1,51 @@
+/*
+Copyright (C) 2022 The Falco Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#pragma once
+
+#include <vector>
+#include <string>
+#include "falco_rule.h"
+#include "indexed_vector.h"
+
+/*!
+	\brief Manager for the internal statistics of the rule engine
+*/
+class stats_manager
+{
+public:
+	/*!
+		\brief Erases the internal state and statistics data
+	*/
+	virtual void clear();
+
+	/*!
+		\brief Callback for when a rule with a given index matches an event
+	*/
+	virtual void on_event(
+		indexed_vector<falco_rule>& rules, uint32_t index);
+
+	/*!
+		\brief Formats the internal statistics into the out sring
+	*/
+	virtual void format_stats(
+		indexed_vector<falco_rule>& rules, std::string& out);
+
+private:
+	uint64_t m_total;
+	std::vector<uint64_t> m_by_priority;
+	std::vector<uint64_t> m_by_rule_id;
+};