Merge pull request #1309 from tbetbetbe/grpc_ruby_rpc_server_md

src/ruby/.rubocop_todo.yml

src/ruby/lib/grpc/errors.rb

src/ruby/lib/grpc/generic/active_call.rb

src/ruby/lib/grpc/generic/rpc_desc.rb

src/ruby/lib/grpc/generic/rpc_server.rb

src/ruby/spec/generic/rpc_desc_spec.rb

src/ruby/spec/generic/rpc_server_pool_spec.rb

src/ruby/spec/generic/rpc_server_spec.rb