feat(core): Allow to remove a Family from their Registry

Fixes: #498

Author: gjasny

core/include/prometheus/registry.h

@@ -80,6 +80,19 @@ class PROMETHEUS_CPP_CORE_EXPORT Registry : public Collectable {
   /// \return Zero or more metrics and their samples.
   std::vector<MetricFamily> Collect() const override;
 
+  /// \brief Removes a metrics family from the registry.
+  ///
+  /// Please note that this operation invalidates the previously
+  /// returned reference to the Family and all of their added
+  /// metric objects.
+  ///
+  /// \tparam T One of the metric types Counter, Gauge, Histogram or Summary.
+  /// \param family The family to remove
+  ///
+  /// \return True if the family was found and removed.
+  template <typename T>
+  bool Remove(const Family<T>& family);
+
  private:
   template <typename T>
   friend class detail::Builder;

core/src/registry.cc

@@ -151,4 +151,34 @@ template Family<Histogram>& Registry::Add(const std::string& name,
                                           const std::string& help,
                                           const Labels& labels);
 
+template <typename T>
+bool Registry::Remove(const Family<T>& family) {
+  std::lock_guard<std::mutex> lock{mutex_};
+
+  auto& families = GetFamilies<T>();
+  auto same_family = [&family](const std::unique_ptr<Family<T>>& in) {
+    return &family == in.get();
+  };
+
+  auto it = std::find_if(families.begin(), families.end(), same_family);
+  if (it == families.end()) {
+    return false;
+  }
+
+  families.erase(it);
+  return true;
+}
+
+template bool PROMETHEUS_CPP_CORE_EXPORT
+Registry::Remove(const Family<Counter>& family);
+
+template bool PROMETHEUS_CPP_CORE_EXPORT
+Registry::Remove(const Family<Gauge>& family);
+
+template bool PROMETHEUS_CPP_CORE_EXPORT
+Registry::Remove(const Family<Summary>& family);
+
+template bool PROMETHEUS_CPP_CORE_EXPORT
+Registry::Remove(const Family<Histogram>& family);
+
 }  // namespace prometheus

core/tests/registry_test.cc

@@ -41,6 +41,20 @@ TEST(RegistryTest, build_histogram_family) {
   ASSERT_EQ(collected.size(), 1U);
 }
 
+TEST(RegistryTest, unable_to_remove_family) {
+  Family<Counter> family{"name", "help", {}};
+  Registry registry{};
+  EXPECT_FALSE(registry.Remove(family));
+}
+
+TEST(RegistryTest, remove_and_readd_family) {
+  Registry registry{Registry::InsertBehavior::Throw};
+
+  auto& counter = BuildCounter().Name("name").Register(registry);
+  EXPECT_TRUE(registry.Remove(counter));
+  EXPECT_NO_THROW(BuildCounter().Name("name").Register(registry));
+}
+
 TEST(RegistryTest, reject_different_type_than_counter) {
   const auto same_name = std::string{"same_name"};
   Registry registry{};