Make ClientMetadataRef::getAgent() const.

Summary:
As titled.

This method is fine as const.

Reviewed By: sazonovkirill

Differential Revision: D86169383

fbshipit-source-id: 0b97fa5da09c805c9244e5e487e4876aee08d581

Author: Cameron Evans

third-party/thrift/src/thrift/lib/cpp2/server/Cpp2ConnContext.cpp

@@ -95,7 +95,7 @@ folly::Optional<std::string> Cpp2ConnContext::PeerCred::getError() const {
   }
 }
 
-std::optional<std::string_view> ClientMetadataRef::getAgent() {
+std::optional<std::string_view> ClientMetadataRef::getAgent() const {
   if (!md_.agent()) {
     return {};
   }

third-party/thrift/src/thrift/lib/cpp2/server/Cpp2ConnContext.h

@@ -65,7 +65,7 @@ class ClientMetadataRef {
  public:
   explicit ClientMetadataRef(const ClientMetadata& md) : md_(md) {}
   std::optional<std::string_view> getHostname() const;
-  std::optional<std::string_view> getAgent();
+  std::optional<std::string_view> getAgent() const;
   std::optional<std::string_view> getOtherMetadataField(std::string_view key);
   const folly::F14NodeMap<std::string, std::string>& getFields() const;