[PH2][Util] Add variant support to MemoryUsageOf (grpc#40487)

Closes grpc#40487

COPYBARA_INTEGRATE_REVIEW=grpc#40487 from ac-patel:memory_usage 88e25c1
PiperOrigin-RevId: 797602976

Author: ac-patel

src/core/util/memory_usage.h

@@ -263,6 +263,12 @@ size_t MemoryUsageOf(const std::tuple<Args...>& t) {
       [](const auto&... args) { return (MemoryUsageOf(args) + ... + 0); }, t);
 }
 
+// This does not calculate the overhead of the variant itself.
+template <typename... T>
+size_t MemoryUsageOf(const std::variant<T...>& t) {
+  return std::visit([](const auto& x) { return MemoryUsageOf(x); }, t);
+}
+
 }  // namespace grpc_core
 
 #endif  // GRPC_SRC_CORE_UTIL_MEMORY_USAGE_H

test/core/util/memory_usage_test.cc

@@ -88,4 +88,11 @@ TEST(MemoryUsageTest, EscapeHatch) {
   EXPECT_EQ(MemoryUsageOf(Foo()), 12345);
 }
 
+TEST(MemoryUsageTest, Variant) {
+  std::variant<int, std::string> v = 42;
+  std::variant<int, std::string> v2 = "hello";
+  EXPECT_EQ(MemoryUsageOf(v), sizeof(int));
+  EXPECT_GE(MemoryUsageOf(v2), sizeof(std::string) + strlen("hello"));
+}
+
 }  // namespace grpc_core