[monitoring] Add support of Datadog

[uro]

Pull request for issue #714

I skipped unit tests of Datadog integration, I think there's nothing to test.

• Implement the integration
• Confirm that it works properly with Datadog
• Confirm that the logic has sense and it's usable for monitoring purposes

[uro]

Hey @makasim
I confirm that integration works and it's usable from data point of view.

All of the metrics are visible by datadog:

I've build simple dashboard to check if metric types and values works well. For now it satisfying me.

Can you please review this pr?

[makasim]

This looks awesome!

Could you please add some doc?

and register it here https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/monitoring/Resources.php#L30

[uro]

Documentation added 👍
https://github.com/uro/enqueue-dev/blob/feature/174-monitoring-datadog-support/docs/monitoring.md#datadog-storage

And as I see, it's already registered in resources.

Feel free to merge :)

[uro]

@makasim I don't understand why build has failed, I didn't touch stomp or anything. Should I do anything more?

[makasim]

available since 0.9.6

[makasim]

as for tests, it is not your fault, it fails from time to time.