fix: Small perf improvement to generate_r (#1522)

Author: fbogsany

api/benchmarks/id_generation_bench.rb

@@ -35,6 +35,16 @@ def generate_span_id_while
   id
 end
 
+def generate_r(trace_id)
+  x = trace_id[8, 8].unpack1('Q>') | 0x3
+  64 - x.bit_length
+end
+
+def generate_r_in_place(trace_id)
+  x = trace_id.unpack1('Q>', offset: 8) | 0x3
+  64 - x.bit_length
+end
+
 Benchmark.ipsa do |x|
   x.report('generate_trace_id') { generate_trace_id }
   x.report('generate_trace_id_while') { generate_trace_id_while }
@@ -46,3 +56,10 @@ def generate_span_id_while
   x.report('generate_span_id_while') { generate_span_id_while }
   x.compare!
 end
+
+Benchmark.ipsa do |x|
+  trace_id = generate_trace_id
+  x.report('generate_r') { generate_r(trace_id) }
+  x.report('generate_r_in_place') { generate_r_in_place(trace_id) }
+  x.compare!
+end

sdk_experimental/lib/opentelemetry/sdk/trace/samplers/consistent_probability_tracestate.rb

@@ -106,7 +106,7 @@ def decimal(str)
           end
 
           def generate_r(trace_id)
-            x = trace_id[8, 8].unpack1('Q>') | 0x3
+            x = trace_id.unpack1('Q>', offset: 8) | 0x3
             64 - x.bit_length
           end
         end