Optimize RequestsInFlightStatsHandler

Start:
Benchmark_RequestsInFlightStatsHandler_HandleRPC-10    	 5970756	       180.9 ns/op	     224 B/op	       5 allocs/op


With attrs set cache:
Benchmark_RequestsInFlightStatsHandler_HandleRPC-10    	11450550	        98.86 ns/op	      96 B/op	       4 allocs/op


With option cache:
Benchmark_RequestsInFlightStatsHandler_HandleRPC-10    	19660729	        57.06 ns/op	      64 B/op	       2 allocs/op

With option slice cache:
Benchmark_RequestsInFlightStatsHandler_HandleRPC-10    	28201672	        37.45 ns/op	      48 B/op	       1 allocs/op