feat core: make sure that labels are not used with integers, leading to memory corruption

Before this patch a code like `writer["metric"].ValueWithLabels(value, {{"label_name2", 2}, {"label_name3", "value3"}});` was attempting to create a `LabelView{std::string_view{"label_name2", 2}, std::string_view{"label_name3", "value3"}}`. The second string_view constructor was corrupting memory as it treated "label_name3" and "value3" as two iterators.

Tests: протестировано локально
commit_hash:e16d3d45a6d3f768611762a4b2fca596aba8d6c8

Author: apolukhin

core/include/userver/utils/statistics/labels.hpp

@@ -19,16 +19,21 @@ class LabelView final {
     LabelView() = default;
     LabelView(Label&& label) = delete;
     explicit LabelView(const Label& label) noexcept;
-    LabelView(std::string_view name, std::string_view value) noexcept : name_(name), value_(value) {}
+    constexpr LabelView(std::string_view name, std::string_view value) noexcept : name_(name), value_(value) {}
 
-    explicit operator bool() const { return !name_.empty(); }
+    template <class T, std::enable_if_t<std::is_arithmetic_v<T>>* = nullptr>
+    constexpr LabelView(std::string_view, T) {
+        static_assert(sizeof(T) && false, "Labels should not be arithmetic values, only strings!");
+    }
+
+    constexpr explicit operator bool() const { return !name_.empty(); }
 
-    std::string_view Name() const { return name_; }
-    std::string_view Value() const { return value_; }
+    constexpr std::string_view Name() const { return name_; }
+    constexpr std::string_view Value() const { return value_; }
 
 private:
-    std::string_view name_;
-    std::string_view value_;
+    std::string_view name_{};
+    std::string_view value_{};
 };
 
 bool operator<(const LabelView& x, const LabelView& y) noexcept;
@@ -41,6 +46,11 @@ class Label final {
     explicit Label(LabelView view);
     Label(std::string name, std::string value);
 
+    template <class T, std::enable_if_t<std::is_arithmetic_v<T>>* = nullptr>
+    Label(std::string, T) {
+        static_assert(sizeof(T) && false, "Labels should not be arithmetic values, only strings!");
+    }
+
     explicit operator bool() const { return !name_.empty(); }
     explicit operator LabelView() const { return {name_, value_}; }