[CODE HEALTH] Fix cppcoreguidelines-pro-type-const-cast for multi observer metrics.

sdk/include/opentelemetry/sdk/metrics/async_instruments.h

@@ -37,9 +37,9 @@ class ObservableInstrument : public opentelemetry::metrics::ObservableInstrument
   void RemoveCallback(opentelemetry::metrics::ObservableCallbackPtr callback,
                       void *state) noexcept override;
 
-  const InstrumentDescriptor &GetInstrumentDescriptor();
+  const InstrumentDescriptor &GetInstrumentDescriptor() const;
 
-  AsyncWritableMetricStorage *GetMetricStorage();
+  AsyncWritableMetricStorage *GetMetricStorage() const;
 
 private:
   InstrumentDescriptor instrument_descriptor_;

sdk/include/opentelemetry/sdk/metrics/multi_observer_result.h

@@ -27,7 +27,7 @@ class OPENTELEMETRY_EXPORT MultiObserverResult final
   size_t InstrumentCount() const;
   bool HasInstrument(const opentelemetry::metrics::ObservableInstrument *instrument) const;
   void GetInstruments(
-      nostd::function_ref<void(opentelemetry::metrics::ObservableInstrument *)> callback);
+      nostd::function_ref<void(const opentelemetry::metrics::ObservableInstrument *)> callback);
   void Reset();
   void StoreResults(opentelemetry::common::SystemTimestamp collection_ts);
 
@@ -43,7 +43,7 @@ class OPENTELEMETRY_EXPORT MultiObserverResult final
   // This allows us to avoid an unnecessary layer of indirection and a bunch of allocations.
   using ObserverResultDirect =
       nostd::variant<nostd::monostate, ObserverResultT<double>, ObserverResultT<int64_t>>;
-  std::unordered_map<opentelemetry::metrics::ObservableInstrument *, ObserverResultDirect>
+  std::unordered_map<const opentelemetry::metrics::ObservableInstrument *, ObserverResultDirect>
       observer_results_;
 };
 }  // namespace metrics

sdk/src/metrics/async_instruments.cc

@@ -43,12 +43,12 @@ void ObservableInstrument::RemoveCallback(opentelemetry::metrics::ObservableCall
   observable_registry_->RemoveCallback(callback, state, this);
 }
 
-const InstrumentDescriptor &ObservableInstrument::GetInstrumentDescriptor()
+const InstrumentDescriptor &ObservableInstrument::GetInstrumentDescriptor() const
 {
   return instrument_descriptor_;
 }
 
-AsyncWritableMetricStorage *ObservableInstrument::GetMetricStorage()
+AsyncWritableMetricStorage *ObservableInstrument::GetMetricStorage() const
 {
   return storage_.get();
 }

sdk/src/metrics/multi_observer_result.cc

@@ -65,12 +65,11 @@ size_t MultiObserverResult::InstrumentCount() const
 bool MultiObserverResult::HasInstrument(
     const opentelemetry::metrics::ObservableInstrument *instrument) const
 {
-  return observer_results_.find(const_cast<opentelemetry::metrics::ObservableInstrument *>(
-             instrument)) != observer_results_.end();
+  return observer_results_.find(instrument) != observer_results_.end();
 }
 
 void MultiObserverResult::GetInstruments(
-    nostd::function_ref<void(opentelemetry::metrics::ObservableInstrument *)> callback)
+    nostd::function_ref<void(const opentelemetry::metrics::ObservableInstrument *)> callback)
 {
   for (auto &el : observer_results_)
   {
@@ -93,8 +92,9 @@ void MultiObserverResult::StoreResults(opentelemetry::common::SystemTimestamp co
     auto *instrument = el.first;
     auto &result     = el.second;
 
-    auto storage = static_cast<opentelemetry::sdk::metrics::ObservableInstrument *>(instrument)
-                       ->GetMetricStorage();
+    auto storage =
+        static_cast<const opentelemetry::sdk::metrics::ObservableInstrument *>(instrument)
+            ->GetMetricStorage();
     nostd::visit(StoreResultVisitor{storage, collection_ts}, result);
   }
 }
@@ -103,10 +103,7 @@ opentelemetry::metrics::ObserverResultT<double> &MultiObserverResult::ForInstrum
     const opentelemetry::metrics::ObservableInstrument *instrument)
 {
   static opentelemetry::sdk::metrics::ObserverResultT<double> null_result;
-  // const_cast is appropriate here, because we're _not_ modifying the passed-in pointer;
-  // we just need to make it non-const to be able to look it up in our map.
-  auto it = observer_results_.find(
-      const_cast<opentelemetry::metrics::ObservableInstrument *>(instrument));
+  auto it = observer_results_.find(instrument);
   if (it == observer_results_.end())
   {
     OTEL_INTERNAL_LOG_ERROR("[MultiObserverResult::ForInstrumentDouble]"
@@ -128,8 +125,7 @@ opentelemetry::metrics::ObserverResultT<int64_t> &MultiObserverResult::ForInstru
     const opentelemetry::metrics::ObservableInstrument *instrument)
 {
   static opentelemetry::sdk::metrics::ObserverResultT<int64_t> null_result;
-  auto it = observer_results_.find(
-      const_cast<opentelemetry::metrics::ObservableInstrument *>(instrument));
+  auto it = observer_results_.find(instrument);
   if (it == observer_results_.end())
   {
     OTEL_INTERNAL_LOG_ERROR("[MultiObserverResult::ForInstrumentInt64]"

sdk/src/metrics/state/observable_registry.cc

@@ -162,9 +162,9 @@ struct InvokeCallbackVisitor
   void operator()(const opentelemetry::metrics::ObservableCallbackPtr &callback)
   {
     record->observable_result.GetInstruments(
-        [&](opentelemetry::metrics::ObservableInstrument *instrument) {
+        [&](const opentelemetry::metrics::ObservableInstrument *instrument) {
           auto value_type =
-              static_cast<opentelemetry::sdk::metrics::ObservableInstrument *>(instrument)
+              static_cast<const opentelemetry::sdk::metrics::ObservableInstrument *>(instrument)
                   ->GetInstrumentDescriptor()
                   .value_type_;
           if (value_type == InstrumentValueType::kDouble)
@@ -185,7 +185,7 @@ struct InvokeCallbackVisitor
   template <typename T>
   void invoke_single_instrument_callback(
       const opentelemetry::metrics::ObservableCallbackPtr &callback,
-      opentelemetry::metrics::ObservableInstrument *instrument)
+      const opentelemetry::metrics::ObservableInstrument *instrument)
   {
     // This is all a bit strangely shaped, but it's in the name of back-compat.
     // The signature of ObservableCallbackPtr is that it takes a nostd::shared_ptr to