[channel-init] Add an escape hatch to have v2-only filters (#36225)

Closes #36225

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36225 from ctiller:skipv3 b4857c3019
PiperOrigin-RevId: 621245586

src/core/ext/filters/channel_idle/legacy_channel_idle_filter.cc

@@ -296,13 +296,13 @@ const grpc_channel_filter LegacyMaxAgeFilter::kFilter =
 
 void RegisterLegacyChannelIdleFilters(CoreConfiguration::Builder* builder) {
   builder->channel_init()
-      ->RegisterFilter<LegacyClientIdleFilter>(GRPC_CLIENT_CHANNEL)
+      ->RegisterV2Filter<LegacyClientIdleFilter>(GRPC_CLIENT_CHANNEL)
       .ExcludeFromMinimalStack()
       .If([](const ChannelArgs& channel_args) {
         return GetClientIdleTimeout(channel_args) != Duration::Infinity();
       });
   builder->channel_init()
-      ->RegisterFilter<LegacyMaxAgeFilter>(GRPC_SERVER_CHANNEL)
+      ->RegisterV2Filter<LegacyMaxAgeFilter>(GRPC_SERVER_CHANNEL)
       .ExcludeFromMinimalStack()
       .If([](const ChannelArgs& channel_args) {
         return LegacyMaxAgeFilter::Config::FromChannelArgs(channel_args)

src/core/lib/surface/channel_init.cc

@@ -141,9 +141,9 @@ ChannelInit::StackConfig ChannelInit::BuildStackConfig(
       GPR_ASSERT(registration->after_.empty());
       GPR_ASSERT(registration->before_.empty());
       GPR_ASSERT(!registration->before_all_);
-      terminal_filters.emplace_back(registration->filter_, nullptr,
-                                    std::move(registration->predicates_),
-                                    registration->registration_source_);
+      terminal_filters.emplace_back(
+          registration->filter_, nullptr, std::move(registration->predicates_),
+          registration->skip_v3_, registration->registration_source_);
     } else {
       dependencies[registration->filter_];  // Ensure it's in the map.
     }
@@ -223,9 +223,9 @@ ChannelInit::StackConfig ChannelInit::BuildStackConfig(
   while (!dependencies.empty()) {
     auto filter = take_ready_dependency();
     auto* registration = filter_to_registration[filter];
-    filters.emplace_back(filter, registration->vtable_,
-                         std::move(registration->predicates_),
-                         registration->registration_source_);
+    filters.emplace_back(
+        filter, registration->vtable_, std::move(registration->predicates_),
+        registration->skip_v3_, registration->registration_source_);
     for (auto& p : dependencies) {
       p.second.erase(filter);
     }
@@ -414,6 +414,7 @@ absl::StatusOr<ChannelInit::StackSegment> ChannelInit::CreateStackSegment(
   size_t channel_data_alignment = 0;
   // Based on predicates build a list of filters to include in this segment.
   for (const auto& filter : stack_config.filters) {
+    if (filter.skip_v3) continue;
     if (!filter.CheckPredicates(args)) continue;
     if (filter.vtable == nullptr) {
       return absl::InvalidArgumentError(

src/core/lib/surface/channel_init.h

@@ -162,6 +162,10 @@ class ChannelInit {
     // Add a predicate that ensures this filter does not appear in the minimal
     // stack.
     FilterRegistration& ExcludeFromMinimalStack();
+    FilterRegistration& SkipV3() {
+      skip_v3_ = true;
+      return *this;
+    }
 
    private:
     friend class ChannelInit;
@@ -172,6 +176,7 @@ class ChannelInit {
     std::vector<InclusionPredicate> predicates_;
     bool terminal_ = false;
     bool before_all_ = false;
+    bool skip_v3_ = false;
     SourceLocation registration_source_;
   };
 
@@ -195,6 +200,15 @@ class ChannelInit {
                             registration_source);
     }
 
+    // Filter does not participate in v3
+    template <typename Filter>
+    FilterRegistration& RegisterV2Filter(
+        grpc_channel_stack_type type, SourceLocation registration_source = {}) {
+      return RegisterFilter(type, &Filter::kFilter, nullptr,
+                            registration_source)
+          .SkipV3();
+    }
+
     // Register a post processor for the builder.
     // These run after the main graph has been placed into the builder.
     // At most one filter per slot per channel stack type can be added.
@@ -274,16 +288,18 @@ class ChannelInit {
  private:
   struct Filter {
     Filter(const grpc_channel_filter* filter, const ChannelFilterVtable* vtable,
-           std::vector<InclusionPredicate> predicates,
+           std::vector<InclusionPredicate> predicates, bool skip_v3,
            SourceLocation registration_source)
         : filter(filter),
           vtable(vtable),
           predicates(std::move(predicates)),
-          registration_source(registration_source) {}
+          registration_source(registration_source),
+          skip_v3(skip_v3) {}
     const grpc_channel_filter* filter;
     const ChannelFilterVtable* vtable;
     std::vector<InclusionPredicate> predicates;
     SourceLocation registration_source;
+    bool skip_v3 = false;
     bool CheckPredicates(const ChannelArgs& args) const;
   };
   struct StackConfig {