Use `ActiveSupport::Notifications::Instrument` instead of `event_listener_proc`

[heka1024]

What? Why?

How about use ActiveSupport::Notifications::Instrument to publish thread_pool_exhausted and unimplemented error?

Many Ruby developers are more familiar with the interface of ActiveSupport::Notifications::Instrument, and it'll make it easier to use with multiple listeners. Additionally, this approach allows us to simplify the implementation in these lines:

gruf/lib/gruf/server.rb

Lines 69 to 74 in c4c4d9e

	server = if @event_listener_proc
	server_options[:event_listener_proc] = @event_listener_proc
	Gruf::InstrumentableGrpcServer.new(**server_options)
	else
	GRPC::RpcServer.new(**server_options)
	end

Since we already have activesupport as a dependency, we can use it without adding any external dependencies.

gruf/gruf.gemspec

Line 60 in 7ce8f7a

spec.add_runtime_dependency 'activesupport', '> 4'