use std::get with bigtable metrics

Author: scotthart

google/cloud/bigtable/internal/operation_context_factory.cc

@@ -214,15 +214,15 @@ void MetricsOperationContextFactory::InitializeProvider(
         auto& labels = *resource.mutable_labels();
         auto const& attributes = pda.attributes.GetAttributes();
         labels[kProjectLabel] =
-            absl::get<std::string>(attributes.find(kProjectLabel)->second);
+            std::get<std::string>(attributes.find(kProjectLabel)->second);
         labels[kInstanceLabel] =
-            absl::get<std::string>(attributes.find(kInstanceLabel)->second);
+            std::get<std::string>(attributes.find(kInstanceLabel)->second);
         labels[kTableLabel] =
-            absl::get<std::string>(attributes.find(kTableLabel)->second);
+            std::get<std::string>(attributes.find(kTableLabel)->second);
         labels[kClusterLabel] =
-            absl::get<std::string>(attributes.find(kClusterLabel)->second);
+            std::get<std::string>(attributes.find(kClusterLabel)->second);
         labels[kZoneLabel] =
-            absl::get<std::string>(attributes.find(kZoneLabel)->second);
+            std::get<std::string>(attributes.find(kZoneLabel)->second);
         return std::make_pair(labels[kProjectLabel], resource);
       };