Add offloop metrics to ThriftServerStats

Summary:
- Thrift java applications extensively use offloop threads
 - It is difficult to quickly triage saturation of offloop threads
 - OffLoopScheduler already has a helper which provides these stats
 - Adding these stats to standard thrift server metrics so these will be available by default to all thrift servers

Reviewed By: avalonalex

Differential Revision: D52928066

fbshipit-source-id: d7449a0bab8d9b4f7b4457657d45967cacedb95b

Author: Vishwa Prakash Gayasen

thrift/lib/java/runtime/src/main/java/com/facebook/swift/service/stats/ServerStats.java

@@ -17,6 +17,7 @@
 package com.facebook.swift.service.stats;
 
 import com.facebook.nifty.core.NiftyMetrics;
+import com.facebook.thrift.util.resources.RpcResources;
 import io.airlift.stats.DecayCounter;
 import io.airlift.stats.Distribution;
 import io.airlift.stats.ExponentialDecay;
@@ -269,6 +270,9 @@ public Map<String, Long> getCounters() {
         (key, value) -> {
           counters.put(THRIFT + key, Math.round(value.getAvg()));
         });
+
+    // Add counters for the off loop scheduler
+    counters.putAll(RpcResources.getOffLoopSchedulerStats());
     return counters;
   }