[server] Make SetRegisteredMethodOnMetadata a method, not a static member fn (#35221)

As part of the call-v3 work I'll be making a call to this code via a different path shortly, and separating the C-style callback piece out helps that work

Closes #35221

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35221 from ctiller:cg-registered-method 4b6d80ee7f
PiperOrigin-RevId: 588200784

src/core/lib/surface/server.cc

@@ -1331,7 +1331,10 @@ void Server::ChannelData::InitTransport(RefCountedPtr<Server> server,
   op->set_accept_stream = true;
   op->set_accept_stream_fn = AcceptStream;
   if (IsRegisteredMethodLookupInTransportEnabled()) {
-    op->set_registered_method_matcher_fn = SetRegisteredMethodOnMetadata;
+    op->set_registered_method_matcher_fn = [](void* arg,
+                                              ClientMetadata* metadata) {
+      static_cast<ChannelData*>(arg)->SetRegisteredMethodOnMetadata(*metadata);
+    };
   }
   // op->set_registered_method_matcher_fn = Registered
   op->set_accept_stream_user_data = this;
@@ -1387,30 +1390,29 @@ Server::ChannelRegisteredMethod* Server::ChannelData::GetRegisteredMethod(
 }
 
 void Server::ChannelData::SetRegisteredMethodOnMetadata(
-    void* arg, ServerMetadata* metadata) {
-  auto* chand = static_cast<Server::ChannelData*>(arg);
-  auto* authority = metadata->get_pointer(HttpAuthorityMetadata());
+    ClientMetadata& metadata) {
+  auto* authority = metadata.get_pointer(HttpAuthorityMetadata());
   if (authority == nullptr) {
-    authority = metadata->get_pointer(HostMetadata());
+    authority = metadata.get_pointer(HostMetadata());
     if (authority == nullptr) {
       // Authority not being set is an RPC error.
       return;
     }
   }
-  auto* path = metadata->get_pointer(HttpPathMetadata());
+  auto* path = metadata.get_pointer(HttpPathMetadata());
   if (path == nullptr) {
     // Path not being set would result in an RPC error.
     return;
   }
   ChannelRegisteredMethod* method;
   if (!IsRegisteredMethodsMapEnabled()) {
-    method = chand->GetRegisteredMethod(authority->c_slice(), path->c_slice());
+    method = GetRegisteredMethod(authority->c_slice(), path->c_slice());
   } else {
-    method = chand->GetRegisteredMethod(authority->as_string_view(),
-                                        path->as_string_view());
+    method = GetRegisteredMethod(authority->as_string_view(),
+                                 path->as_string_view());
   }
   // insert in metadata
-  metadata->Set(GrpcRegisteredMethod(), method);
+  metadata.Set(GrpcRegisteredMethod(), method);
 }
 
 void Server::ChannelData::AcceptStream(void* arg, Transport* /*transport*/,

src/core/lib/surface/server.h

@@ -268,8 +268,7 @@ class Server : public InternallyRefCounted<Server>,
 
     static void AcceptStream(void* arg, Transport* /*transport*/,
                              const void* transport_server_data);
-    static void SetRegisteredMethodOnMetadata(void* arg,
-                                              ServerMetadata* metadata);
+    void SetRegisteredMethodOnMetadata(ClientMetadata& metadata);
 
     void Destroy() ABSL_EXCLUSIVE_LOCKS_REQUIRED(server_->mu_global_);