[SDK] support aggregation of identical instruments

CHANGELOG.md

@@ -39,6 +39,9 @@ Increment the:
 * [SDK] Optimize PeriodicExportingMetricReader thread usage
   [#3383](https://github.com/open-telemetry/opentelemetry-cpp/pull/3383)
 
+* [SDK] Aggregate identical metrics instruments and detect duplicates
+  [#3358](https://github.com/open-telemetry/opentelemetry-cpp/pull/3358)
+
 ## [1.20 2025-04-01]
 
 * [BUILD] Update opentelemetry-proto version

sdk/include/opentelemetry/sdk/metrics/instruments.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <algorithm>
+#include <cctype>
 #include <functional>
 #include <string>
 
@@ -65,6 +67,152 @@ struct InstrumentDescriptor
   InstrumentValueType value_type_;
 };
 
+struct InstrumentDescriptorUtil
+{
+  // Case-insensitive comparison of two ASCII strings used to evaluate equality of instrument names
+  static bool CaseInsensitiveAsciiEquals(const std::string &lhs, const std::string &rhs) noexcept
+  {
+    return lhs.size() == rhs.size() &&
+           std::equal(lhs.begin(), lhs.end(), rhs.begin(), [](char a, char b) {
+             return std::tolower(static_cast<unsigned char>(a)) ==
+                    std::tolower(static_cast<unsigned char>(b));
+           });
+  }
+
+  // Implementation of the specification requirements on duplicate instruments
+  // An instrument is a duplicate if it has the same name (case-insensitive) as another instrument,
+  // but different instrument kind, unit, or description.
+  // https://github.com/open-telemetry/opentelemetry-specification/blob/9c8c30631b0e288de93df7452f91ed47f6fba330/specification/metrics/sdk.md?plain=1#L869
+  static bool IsDuplicate(const InstrumentDescriptor &lhs, const InstrumentDescriptor &rhs) noexcept
+  {
+    // Not a duplicate if case-insensitive names are not equal
+    if (!InstrumentDescriptorUtil::CaseInsensitiveAsciiEquals(lhs.name_, rhs.name_))
+    {
+      return false;
+    }
+
+    // Duplicate if names equal and kinds (Type and ValueType) are not equal
+    if (lhs.type_ != rhs.type_ || lhs.value_type_ != rhs.value_type_)
+    {
+      return true;
+    }
+
+    // Duplicate if names equal and units (case-sensitive) are not equal
+    if (lhs.unit_ != rhs.unit_)
+    {
+      return true;
+    }
+
+    // Duplicate if names equal and descriptions (case-sensitive) are not equal
+    if (lhs.description_ != rhs.description_)
+    {
+      return true;
+    }
+
+    // All identifying fields are equal
+    // These are identical instruments or only have a name case conflict
+    return false;
+  }
+
+  static opentelemetry::nostd::string_view GetInstrumentTypeString(InstrumentType type) noexcept
+  {
+    switch (type)
+    {
+      case InstrumentType::kCounter:
+        return "Counter";
+      case InstrumentType::kUpDownCounter:
+        return "UpDownCounter";
+      case InstrumentType::kHistogram:
+        return "Histogram";
+      case InstrumentType::kObservableCounter:
+        return "ObservableCounter";
+      case InstrumentType::kObservableUpDownCounter:
+        return "ObservableUpDownCounter";
+      case InstrumentType::kObservableGauge:
+        return "ObservableGauge";
+      case InstrumentType::kGauge:
+        return "Gauge";
+      default:
+        return "Unknown";
+    }
+  }
+
+  static opentelemetry::nostd::string_view GetInstrumentValueTypeString(
+      InstrumentValueType value_type) noexcept
+  {
+    switch (value_type)
+    {
+      case InstrumentValueType::kInt:
+        return "Int";
+      case InstrumentValueType::kLong:
+        return "Long";
+      case InstrumentValueType::kFloat:
+        return "Float";
+      case InstrumentValueType::kDouble:
+        return "Double";
+      default:
+        return "Unknown";
+    }
+  }
+};
+
+struct InstrumentEqualNameCaseInsensitive
+{
+  bool operator()(const InstrumentDescriptor &lhs, const InstrumentDescriptor &rhs) const noexcept
+  {
+    // Names (case-insensitive)
+    if (!InstrumentDescriptorUtil::CaseInsensitiveAsciiEquals(lhs.name_, rhs.name_))
+    {
+      return false;
+    }
+
+    // Kinds (Type and ValueType)
+    if (lhs.type_ != rhs.type_ || lhs.value_type_ != rhs.value_type_)
+    {
+      return false;
+    }
+
+    // Units (case-sensitive)
+    if (lhs.unit_ != rhs.unit_)
+    {
+      return false;
+    }
+
+    // Descriptions (case-sensitive)
+    if (lhs.description_ != rhs.description_)
+    {
+      return false;
+    }
+
+    // All identifying fields are equal
+    return true;
+  }
+};
+
+// Hash for InstrumentDescriptor
+// Identical instruments must have the same hash value
+// Two instruments are identical when all identifying fields (case-insensitive name , kind,
+// description, unit) are equal.
+struct InstrumentDescriptorHash
+{
+  std::size_t operator()(const InstrumentDescriptor &instrument_descriptor) const noexcept
+  {
+    std::size_t hashcode{};
+
+    for (char c : instrument_descriptor.name_)
+    {
+      sdk::common::GetHash(hashcode,
+                           static_cast<char>(std::tolower(static_cast<unsigned char>(c))));
+    }
+
+    sdk::common::GetHash(hashcode, instrument_descriptor.description_);
+    sdk::common::GetHash(hashcode, instrument_descriptor.unit_);
+    sdk::common::GetHash(hashcode, static_cast<uint32_t>(instrument_descriptor.type_));
+    sdk::common::GetHash(hashcode, static_cast<uint32_t>(instrument_descriptor.value_type_));
+    return hashcode;
+  }
+};
+
 using MetricAttributes               = opentelemetry::sdk::metrics::FilteredOrderedAttributeMap;
 using MetricAttributesHash           = opentelemetry::sdk::metrics::FilteredOrderedAttributeMapHash;
 using AggregationTemporalitySelector = std::function<AggregationTemporality(InstrumentType)>;

sdk/include/opentelemetry/sdk/metrics/meter.h

@@ -136,8 +136,12 @@ class Meter final : public opentelemetry::metrics::Meter
   // meter-context.
   std::unique_ptr<sdk::instrumentationscope::InstrumentationScope> scope_;
   std::weak_ptr<sdk::metrics::MeterContext> meter_context_;
-  // Mapping between instrument-name and Aggregation Storage.
-  std::unordered_map<std::string, std::shared_ptr<MetricStorage>> storage_registry_;
+  // Mapping between instrument descriptor and Aggregation Storage.
+  using MetricStorageMap = std::unordered_map<InstrumentDescriptor,
+                                              std::shared_ptr<MetricStorage>,
+                                              InstrumentDescriptorHash,
+                                              InstrumentEqualNameCaseInsensitive>;
+  MetricStorageMap storage_registry_;
   std::shared_ptr<ObservableRegistry> observable_registry_;
   MeterConfig meter_config_;
   std::unique_ptr<SyncWritableMetricStorage> RegisterSyncMetricStorage(
@@ -164,6 +168,19 @@ class Meter final : public opentelemetry::metrics::Meter
     return instrument_validator.ValidateName(name) && instrument_validator.ValidateUnit(unit) &&
            instrument_validator.ValidateDescription(description);
   }
+
+  // This function checks if the instrument is a duplicate of an existing one
+  // and emits a warning through the internal logger.
+  static void WarnOnDuplicateInstrument(
+      const sdk::instrumentationscope::InstrumentationScope *scope,
+      const MetricStorageMap &storage_registry,
+      const InstrumentDescriptor &new_instrument);
+
+  // This function checks if the instrument has a name case conflict with an existing one
+  // and emits a warning through the internal logger.
+  static void WarnOnNameCaseConflict(const sdk::instrumentationscope::InstrumentationScope *scope,
+                                     const InstrumentDescriptor &existing_instrument,
+                                     const InstrumentDescriptor &new_instrument);
 };
 }  // namespace metrics
 }  // namespace sdk