Add credits_available quantile counter (p1, p50, p99)

Summary: Add quantile counter (p1, p50, and p99) for number of stream credits the server currently has. The value is updated each time the server receives more credits or if it runs out of credits.

Reviewed By: praihan

Differential Revision: D68467565

fbshipit-source-id: ecc316562572badf057b94fb460af4164e6c4f4b

thrift/lib/cpp2/server/metrics/StreamMetricCallback.h

@@ -31,6 +31,8 @@ class StreamMetricCallback {
   virtual void onStreamCancel(std::string_view /* methodName */) = 0;
   virtual void onStreamRequestN(
       std::string_view /* methodName */, uint32_t /* credits */) = 0;
+  virtual void recordCreditsAvailable(
+      std::string_view /* methodName */, uint32_t /* creditsAvailable */) = 0;
 };
 
 class NoopStreamMetricCallback final : public StreamMetricCallback {
@@ -41,6 +43,7 @@ class NoopStreamMetricCallback final : public StreamMetricCallback {
   void onStreamComplete(std::string_view) override {}
   void onStreamCancel(std::string_view) override {}
   void onStreamRequestN(std::string_view, uint32_t) override {}
+  void recordCreditsAvailable(std::string_view, uint32_t) override {}
 };
 
 } // namespace apache::thrift

thrift/lib/cpp2/transport/rocket/server/RocketStreamClientCallback.cpp

@@ -226,8 +226,9 @@ bool RocketStreamClientCallback::request(uint32_t tokens) {
   }
 
   cancelTimeout();
-  tokens_ += tokens;
+  streamMetricCallback_.recordCreditsAvailable(rpcMethodName_, tokens_);
   streamMetricCallback_.onStreamRequestN(rpcMethodName_, tokens);
+  tokens_ += tokens;
   return serverCallback()->onStreamRequestN(tokens);
 }
 
@@ -292,6 +293,7 @@ void RocketStreamClientCallback::scheduleTimeout() {
     timeoutCallback_ = std::make_unique<TimeoutCallback>(*this);
   }
   connection_.scheduleStreamTimeout(timeoutCallback_.get());
+  streamMetricCallback_.recordCreditsAvailable(rpcMethodName_, tokens_);
 }
 
 void RocketStreamClientCallback::cancelTimeout() {