Add process collector

src/metric.hpp

@@ -45,6 +45,7 @@
 
 #include <functional>
 #include <initializer_list>
+#include <memory>
 #include <span>
 #include <type_traits>
 #include <vector>
@@ -231,6 +232,36 @@ bool is_valid_label_value(std::string_view value);
 // when no metric objects are in use and no concurrent render() calls.
 void reset_metrics_for_testing();
 
+/**
+ * @brief Interface for a custom collector that can be registered with the
+ * metrics system.
+ *
+ * This is used for complex metric gathering, such as reading from /proc, where
+ * multiple metrics need to be updated from a single data source.
+ */
+struct Collector {
+  /**
+   * @brief Virtual destructor.
+   */
+  virtual ~Collector() = default;
+
+  /**
+   * @brief Called by the metrics system to update the metrics this collector is
+   * responsible for.
+   */
+  virtual void collect() = 0;
+};
+
+/**
+ * @brief Register a collector with the metrics system.
+ *
+ * The system will hold a shared_ptr to the collector and call its collect()
+ * method during each metric rendering.
+ *
+ * @param collector A shared_ptr to the collector to be registered.
+ */
+void register_collector(std::shared_ptr<Collector> collector);
+
 // Note: Histograms do not support callbacks due to their multi-value nature
 // (buckets + sum + count). Use static histogram metrics only.