Use Concurrent.usable_processor_count when it is available

It's a new API introduced in concurrent-ruby 1.3.1, which works better
in the container environment.

ruby-concurrency/concurrent-ruby#1038

Since there are gems like sorbet-runtime that still use older versions
of concurrent-ruby, we can't directly bump concurrent-ruby's requirement,
but need to check if the method is available before calling it.

Author: st0012

sentry-ruby/lib/sentry/configuration.rb

@@ -351,7 +351,7 @@ def add_post_initialization_callback(&block)
     def initialize
       self.app_dirs_pattern = nil
       self.debug = false
-      self.background_worker_threads = (Concurrent.processor_count / 2.0).ceil
+      self.background_worker_threads = (processor_count / 2.0).ceil
       self.background_worker_max_queue = BackgroundWorker::DEFAULT_MAX_QUEUE
       self.backtrace_cleanup_callback = nil
       self.max_breadcrumbs = BreadcrumbBuffer::DEFAULT_SIZE
@@ -654,5 +654,13 @@ def run_post_initialization_callbacks
         instance_eval(&hook)
       end
     end
+
+    def processor_count
+      if Concurrent.respond_to?(:usable_processor_count)
+        Concurrent.usable_processor_count
+      else
+        Concurrent.processor_count
+      end
+    end
   end
 end

sentry-ruby/spec/sentry/configuration_spec.rb

@@ -26,12 +26,12 @@
 
   describe "#background_worker_threads" do
     it "sets to have of the processors count" do
-      allow(Concurrent).to receive(:processor_count).and_return(8)
+      allow_any_instance_of(Sentry::Configuration).to receive(:processor_count).and_return(8)
       expect(subject.background_worker_threads).to eq(4)
     end
 
     it "sets to 1 with only 1 processor" do
-      allow(Concurrent).to receive(:processor_count).and_return(1)
+      allow_any_instance_of(Sentry::Configuration).to receive(:processor_count).and_return(1)
       expect(subject.background_worker_threads).to eq(1)
     end
   end