Fix a potential precision loss on integer in ReservoirCellIndexFor (#1696)

Author: ThomsonTan

sdk/include/opentelemetry/sdk/metrics/exemplar/histogram_exemplar_reservoir.h

@@ -55,12 +55,12 @@ class HistogramExemplarReservoir : public FixedSizeExemplarReservoir
                               const MetricAttributes & /* attributes */,
                               const opentelemetry::context::Context & /* context */) override
     {
-      int max_size = boundaries_.size();
-      for (int i = 0; i < max_size; ++i)
+      size_t max_size = boundaries_.size();
+      for (size_t i = 0; i < max_size; ++i)
       {
         if (value <= boundaries_[i])
         {
-          return i;
+          return static_cast<int>(i);
         }
       }
       return -1;