Add ODS counter for fschannel task queue

Summary:
I am unbounding the fschannelthread task queue due to a deadlock (see stack)
This adds a counter for the number of tasks queued up for the fschannelthreads.

We will monitor this when we unbound to see how much memory we are using and
ensuring that things are staying reasonable. This will also help inform how
many pending requests we should allow max.

Note that because we don't have ODS on some platforms we will rely on the logging
in D56739177 more heavily for making the max decision.

Reviewed By: MichaelCuevas

Differential Revision: D56739143

fbshipit-source-id: 9c5a17a2a41ce9d9791a04d5154702bf292d1e6e

eden/common/utils/UnboundedQueueExecutor.cpp

@@ -27,4 +27,14 @@ UnboundedQueueExecutor::UnboundedQueueExecutor(
     std::shared_ptr<folly::ManualExecutor> executor)
     : executor_{std::move(executor)} {}
 
+size_t UnboundedQueueExecutor::getTaskQueueSize() const {
+  if (auto ex =
+          std::dynamic_pointer_cast<folly::CPUThreadPoolExecutor>(executor_)) {
+    return ex->getTaskQueueSize();
+  }
+  // manual executor does not expose a way to count the number of pending
+  // tasks.
+  return 0;
+}
+
 } // namespace facebook::eden

eden/common/utils/UnboundedQueueExecutor.h

@@ -50,6 +50,8 @@ class UnboundedQueueExecutor : public folly::Executor {
     executor_->add(std::move(func));
   }
 
+  size_t getTaskQueueSize() const;
+
  private:
   std::shared_ptr<folly::Executor> executor_;
 };