Revert to non const map with const return

include/grpcpp/security/alts_context.h

@@ -51,7 +51,7 @@ class AltsContext {
   std::string local_service_account() const;
   grpc_security_level security_level() const;
   RpcProtocolVersions peer_rpc_versions() const;
-  const std::map<std::string, std::string> peer_attributes() const;
+  std::map<std::string, std::string> peer_attributes() const;
 
  private:
   std::string application_protocol_;

src/cpp/common/alts_context.cc

@@ -119,7 +119,7 @@ AltsContext::RpcProtocolVersions AltsContext::peer_rpc_versions() const {
   return peer_rpc_versions_;
 }
 
-const std::map<std::string, std::string> AltsContext::peer_attributes() const {
+std::map<std::string, std::string> AltsContext::peer_attributes() const{
   return peer_attributes_map_;
 }
 

test/cpp/common/alts_util_test.cc

@@ -128,7 +128,7 @@ TEST(AltsUtilTest, AuthContextWithGoodAltsContextWithoutRpcVersions) {
   EXPECT_EQ(0, rpc_protocol_versions.min_rpc_version.major_version);
   EXPECT_EQ(0, rpc_protocol_versions.min_rpc_version.minor_version);
   EXPECT_EQ(expected_peer_atrributes_value,
-            alts_context->peer_attributes()[expected_peer_atrributes_key]);
+            alts_context->peer_attributes().at(expected_peer_atrributes_key));
 }
 
 TEST(AltsUtilTest, AuthContextWithGoodAltsContext) {