diff --git a/src/metric.cpp b/src/metric.cpp
index 0689aca..4fa03ad 100644
--- a/src/metric.cpp
+++ b/src/metric.cpp
@@ -4,8 +4,35 @@
 #include <thread>
 #include <unordered_map>
 #include <vector>
+namespace metric {
+struct MetricKey {
+  std::string_view name;
+  std::vector<std::pair<std::string, std::string>> labels;
+  bool operator==(const MetricKey &other) const {
+    return name == other.name && labels == other.labels;
+  }
+};
+} // namespace metric
+
+namespace std {
+template <> struct hash<metric::MetricKey> {
+  std::size_t operator()(const metric::MetricKey &k) const {
+    thread_local std::vector<size_t> parts;
+    parts.clear();
+    parts.push_back(std::hash<std::string_view>{}(k.name));
+    for (const auto &p : k.labels) {
+      parts.push_back(std::hash<std::string>{}(p.first));
+      parts.push_back(std::hash<std::string>{}(p.second));
+    }
+    return std::hash<std::string_view>{}(
+        std::string_view{reinterpret_cast<const char *>(parts.data()),
+                         parts.size() * sizeof(size_t)});
+  }
+};
+} // namespace std
 
 namespace metric {
+
 struct Counter::State {};
 struct Gauge::State {};
 struct Histogram::State {};
@@ -13,11 +40,6 @@ struct Metric {
 
   static std::mutex mutex;
 
-  struct MetricKey {
-    std::string_view name;
-    std::vector<std::pair<std::string, std::string>> labels;
-  };
-
   struct PerThreadState {
     std::unordered_map<MetricKey, Counter::State> counters;
     std::unordered_map<MetricKey, Histogram::State> histograms;
diff --git a/src/metric.hpp b/src/metric.hpp
index 25ae7fa..3be5949 100644
--- a/src/metric.hpp
+++ b/src/metric.hpp
@@ -3,8 +3,10 @@
 #include "arena_allocator.hpp"
 #include <initializer_list>
 #include <span>
+#include <string>
 #include <string_view>
 #include <type_traits>
+#include <vector>
 
 namespace metric {