Add ODS counter for idle timeout connection terminations

Summary:
When `ThriftServer::setIdleTimeout()` is configured, connections that remain
idle for the specified duration are terminated via
`RocketServerConnection::timeoutExpired()`. Currently there is no counter
tracking these events. Adding one gives service owners visibility into idle
timeout behavior, following the same pattern as `thrift.dropped_conns`,
`thrift.rejected_conns`, and `thrift.accepted_connections`.

## Implementation

### 1. Add `connClosedByIdleTimeout()` to `TServerObserver`

**File:** `fbcode/thrift/lib/cpp/server/TServerObserver.h`

New virtual method after `connClosed`:
```cpp
virtual void connClosedByIdleTimeout() {}
```

### 2. Add timeseries field + override to `TServerCounters`

**File:** `fbcode/common/fb303/cpp/TServerCounters.h`

- New field `idleTimeoutConns_` (after `rejectedConns_`)
- New override declaration `connClosedByIdleTimeout()`

### 3. Wire up timeseries creation + callback in `TServerCounters.cpp`

**File:** `fbcode/common/fb303/cpp/TServerCounters.cpp`

- In `setupCounters()`: `idleTimeoutConns_ = makeTimeseries(prefix + ".idle_timeout_conns", COUNT);`
- New implementation of `connClosedByIdleTimeout()` that increments the timeseries

### 4. Add `onIdleTimeout()` callback to `RocketServerHandler`

**File:** `fbcode/thrift/lib/cpp2/transport/rocket/server/RocketServerHandler.h`

New virtual method after `connectionClosing()`:
```cpp
virtual void onIdleTimeout() {}
```

### 5. Override in `ThriftRocketServerHandler`

**Files:**
- `ThriftRocketServerHandler.h` — declaration: `void onIdleTimeout() final;`
- `ThriftRocketServerHandler.cpp` — implementation calls `observer->connClosedByIdleTimeout()`

### 6. Call from `RocketServerConnection::timeoutExpired()`

**File:** `fbcode/thrift/lib/cpp2/transport/rocket/server/RocketServerConnection.cpp`

Added `frameHandler_->onIdleTimeout()` call before `closeWhenIdle()` in `timeoutExpired()`.

### 7. Update `FakeServerObserver` for testing

**File:** `fbcode/thrift/lib/cpp2/transport/core/testutil/FakeServerObserver.h`

- New atomic counter `connClosedByIdleTimeout_`
- New override `connClosedByIdleTimeout()` that increments the counter

### 8. Add test verification

**File:** `fbcode/thrift/lib/cpp2/test/server/ThriftServerTest.cpp`

Updated `ConnectionIdleTimeoutTest` to attach a `FakeServerObserver`, verify
the counter is 0 before idle timeout, and verify it increments to 1 after the
connection is terminated.

## ODS Counter Name

The counter will be published as `thrift.idle_timeout_conns` (with aggregation
suffixes like `.count.60`, `.count.600`, `.count.3600`).

## Files Modified

| File | Change |
|------|--------|
| `thrift/lib/cpp/server/TServerObserver.h` | New virtual method |
| `common/fb303/cpp/TServerCounters.h` | New field + override decl |
| `common/fb303/cpp/TServerCounters.cpp` | Create timeseries + impl |
| `thrift/lib/cpp2/transport/rocket/server/RocketServerHandler.h` | New virtual method |
| `thrift/lib/cpp2/transport/rocket/server/ThriftRocketServerHandler.h` | Override decl |
| `thrift/lib/cpp2/transport/rocket/server/ThriftRocketServerHandler.cpp` | Override impl |
| `thrift/lib/cpp2/transport/rocket/server/RocketServerConnection.cpp` | Call onIdleTimeout() |
| `thrift/lib/cpp2/transport/core/testutil/FakeServerObserver.h` | New counter + override |
| `thrift/lib/cpp2/test/server/ThriftServerTest.cpp` | Verify counter in test |
| `thrift/lib/cpp2/test/BUCK` | Add mocks dep |

Reviewed By: sazonovkirill

Differential Revision: D95074970

fbshipit-source-id: e263c7b46be62b8e46aa7ec41e23451d19ef718a

Author: Fadi Hanna

third-party/thrift/src/thrift/lib/cpp/server/TServerObserver.h

@@ -159,6 +159,8 @@ class TServerObserver {
 
   virtual void connClosed(const ConnectionInfo&) {}
 
+  virtual void connClosedByIdleTimeout() {}
+
   virtual void activeConnections(int32_t /*numConnections*/) {}
 
   virtual void tlsError() {}

third-party/thrift/src/thrift/lib/cpp2/test/server/ThriftServerTest.cpp

@@ -83,6 +83,7 @@
 #include <thrift/lib/cpp2/test/server/ThriftServerTestUtils.h>
 #include <thrift/lib/cpp2/test/util/TestHandler.h>
 #include <thrift/lib/cpp2/test/util/TestThriftServerFactory.h>
+#include <thrift/lib/cpp2/transport/core/testutil/FakeServerObserver.h>
 #include <thrift/lib/cpp2/transport/http2/common/HTTP2RoutingHandler.h>
 #include <thrift/lib/cpp2/util/ScopedServerInterfaceThread.h>
 #include <thrift/lib/cpp2/util/ScopedServerThread.h>
@@ -2315,8 +2316,12 @@ TEST(ThriftServer, ConnectionIdleTimeoutTest) {
   TestThriftServerFactory<TestHandler> factory;
   auto server = factory.create();
   server->setIdleTimeout(std::chrono::milliseconds(20));
+  auto observer = std::make_shared<FakeServerObserver>();
+  server->setObserver(observer);
   apache::thrift::util::ScopedServerThread st(server);
 
+  EXPECT_EQ(observer->connClosedByIdleTimeout_, 0);
+
   folly::EventBase base;
   auto socket = folly::AsyncSocket::newSocket(&base, *st.getAddress());
 
@@ -2327,6 +2332,11 @@ TEST(ThriftServer, ConnectionIdleTimeoutTest) {
   client.sync_sendResponse(response, 200);
   EXPECT_EQ(response, "test200");
   base.loop();
+
+  // Wait for idle timeout to fire and close the connection
+  /* sleep override */
+  std::this_thread::sleep_for(std::chrono::milliseconds(200));
+  EXPECT_EQ(observer->connClosedByIdleTimeout_, 1);
 }
 
 TEST(ThriftServer, BadSendTest) {

third-party/thrift/src/thrift/lib/cpp2/transport/core/testutil/FakeServerObserver.h

@@ -29,6 +29,7 @@ class FakeServerObserver : public apache::thrift::server::TServerObserver {
   std::atomic<size_t> connClosed_{0};
   std::atomic<size_t> connDropped_{0};
   std::atomic<size_t> connRejected_{0};
+  std::atomic<size_t> connClosedByIdleTimeout_{0};
   std::atomic<size_t> activeConns_{0};
   std::atomic<size_t> taskKilled_{0};
   std::atomic<size_t> taskTimeout_{0};
@@ -63,6 +64,8 @@ class FakeServerObserver : public apache::thrift::server::TServerObserver {
 
   void connRejected() override { ++connRejected_; }
 
+  void connClosedByIdleTimeout() override { ++connClosedByIdleTimeout_; }
+
   void activeConnections(int32_t numConnections) override {
     activeConns_ = numConnections;
   }

third-party/thrift/src/thrift/lib/cpp2/transport/rocket/server/RocketServerConnection.cpp

@@ -988,6 +988,7 @@ void RocketServerConnection::timeoutExpired() noexcept {
   DestructorGuard dg(this);
 
   if (!isBusy()) {
+    frameHandler_->onIdleTimeout();
     closeWhenIdle();
   }
 }

third-party/thrift/src/thrift/lib/cpp2/transport/rocket/server/RocketServerHandler.h

@@ -66,6 +66,8 @@ class RocketServerHandler {
 
   virtual void connectionClosing() = 0;
 
+  virtual void onIdleTimeout() {}
+
   virtual Cpp2ConnContext* getCpp2ConnContext() { return nullptr; }
 
   virtual void onBeforeHandleFrame() {}

third-party/thrift/src/thrift/lib/cpp2/transport/rocket/server/ThriftRocketServerHandler.cpp

@@ -563,6 +563,12 @@ void ThriftRocketServerHandler::connectionClosing() {
   }
 }
 
+void ThriftRocketServerHandler::onIdleTimeout() {
+  if (auto* observer = worker_->getServer()->getObserver()) {
+    observer->connClosedByIdleTimeout();
+  }
+}
+
 template <class F>
 void ThriftRocketServerHandler::handleRequestCommon(
     Payload&& payload,

third-party/thrift/src/thrift/lib/cpp2/transport/rocket/server/ThriftRocketServerHandler.h

@@ -95,6 +95,7 @@ class ThriftRocketServerHandler : public RocketServerHandler {
       RocketServerFrameContext&& context,
       ChannelRequestCallbackFactory clientCallback) final;
   void connectionClosing() final;
+  void onIdleTimeout() final;
 
   apache::thrift::server::TServerObserver::SamplingStatus shouldSample(
       const transport::THeader& header);