NewRelic instrumentation async Sinatra. But note that this doesn't instrument EM.callback etc.. You still need custom instrumentation for callbacks.

newrelic_async_sinatra.rb

# NewRelic instrumentation for async_sinatra.
#
# The following code was reffered.
# https://github.com/newrelic/rpm/blob/51767a324046baff0ea4d347862cd4696d73374d/lib/new_relic/agent/instrumentation/sinatra.rb

require 'newrelic_rpm'
require 'new_relic/agent/instrumentation/controller_instrumentation'

module AsyncSinatraInstrumentation
  def async_handle_exception_with_newrelic &blk
    if @monitored
      async_handle_exception_without_newrelic &blk
    else
      @monitored = true
      txn_name = ""
      begin
        txn_name = NewRelic::Agent::Instrumentation::Sinatra::NewRelic.transaction_name(self.class.routes, @request) do |pattern, keys, conditions|
          process_route(pattern, keys, conditions) do
            pattern.source
          end
        end
      rescue
        # For a safe reason.
        txn_name = "UNKNOWN"
      end

      params = @request ? @request.params : {}

      perform_action_with_newrelic_trace(:category => :sinatra,
                                         :name => "A-" + txn_name,
                                         :params => params) do
        async_handle_exception_without_newrelic &blk
      end
    end
  end
end

# With NewRelic Sinatra information, DependencyDetection won't be called after
# required 'newrelic_rpm', so simply put the instrumentation here.

if defined?(::Sinatra) && defined?(::Sinatra::Base) &&
  Sinatra::Base.method_defined?(:dispatch_with_newrelic)

  ::NewRelic::Agent.logger.info 'Installing Async Sinatra instrumentation'

  ::Sinatra::Async::Helpers.class_eval do
    include AsyncSinatraInstrumentation
    alias async_handle_exception_without_newrelic async_handle_exception
    alias async_handle_exception async_handle_exception_with_newrelic
  end
end

# DependencyDetection.defer do
#   @name = :async_sinatra
#
#   depends_on do
#     defined?(::Sinatra::Async) and not NewRelic::Control.instance['disable_async_sinatra']
#   end
#
#   executes do
#     ::NewRelic::Agent.logger.info 'Installing Async Sinatra instrumentation'
#   end
#
#   executes do
#     ::Sinatra::Async::Helpers.class_eval do
#       include AsyncSinatraInstrumentation
#       alias async_handle_exception_without_newrelic async_handle_exception
#       alias async_handle_exception async_handle_exception_with_newrelic
#     end
#   end
# end