chore: add benchmark gem as dependency as from ruby 3.5 it will not be included in default libraries

Signed-off-by: Arjun Rajappa <Arjun.Rajappa@ibm.com>

Author: arjun-rajappa

instana.gemspec

@@ -40,6 +40,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "appraisal"
   spec.add_development_dependency "fakefs"
   spec.add_development_dependency "irb"
+  spec.add_development_dependency "benchmark"
 
   spec.add_runtime_dependency('base64', '>= 0.1')
   spec.add_runtime_dependency('logger')