Let encoders decide whether to encode HttpMethodMetadata or not, instead of removing it. (#28369)

src/core/ext/filters/client_channel/client_channel.cc

@@ -2446,6 +2446,8 @@ class ClientChannel::LoadBalancedCall::Metadata
 
     void Encode(GrpcTimeoutMetadata, grpc_millis) {}
     void Encode(HttpPathMetadata, const Slice&) {}
+    void Encode(HttpMethodMetadata,
+                const typename HttpMethodMetadata::ValueType&) {}
 
     std::vector<std::pair<std::string, std::string>> Take() {
       return std::move(out_);

src/core/ext/filters/http/server/http_server_filter.cc

@@ -120,7 +120,7 @@ static grpc_error_handle hs_filter_incoming_metadata(grpc_call_element* elem,
   grpc_error_handle error = GRPC_ERROR_NONE;
   static const char* error_name = "Failed processing incoming headers";
 
-  auto method = b->Take(grpc_core::HttpMethodMetadata());
+  auto method = b->get(grpc_core::HttpMethodMetadata());
   if (method.has_value()) {
     switch (*method) {
       case grpc_core::HttpMethodMetadata::kPost:

src/core/lib/security/transport/server_auth_filter.cc

@@ -108,6 +108,9 @@ class ArrayEncoder {
         grpc_core::Slice(Which::Encode(value)));
   }
 
+  void Encode(grpc_core::HttpMethodMetadata,
+              const typename grpc_core::HttpMethodMetadata::ValueType&) {}
+
  private:
   void Add(grpc_core::Slice key, grpc_core::Slice value) {
     if (array_->count == array_->capacity) {