statsd: add Flush() to datadogBackend

[mtwstudios]

Remade this PR from #141 to re-trigger CLA

This PR adds a public method to flush any remaining metrics manually (rather than waiting for the flush interval), by calling the Flush() method on the dogstatsd client.

This is helpful in the case of recording a metric right before terminating the program. In this case, the metric will not get submitted to datadog - the only workaround being putting a sleep in place for at least as long as the flush interval.

With this, one can call Flush() before the program terminates to make sure any remaining metrics are submitted to Datadog.

[mtwstudios]

@lavoiesl would you be the right person to review this? Let me know if you have any questions/concerns, etc.

[lavoiesl]

Adding the method here would not be accessible because only the interface is exported.

Adding the method on the interface would be a breaking change.

Not saying it can't be done, but it's a more complicated change than adding it here.

As an alternative could be to add a method to take in a Client such that one could flush the client directly?

func NewDatadogBackend(endpoint, namespace string, tags []string) (Backend, error) {
	client, err := statsd.New(endpoint)
	if err != nil {
		return nil, err
	}
	client.Tags = tags
	return NewDatadogBackendFromClient(client, namespace)
}

func NewDatadogBackendFromClient(client *statsd.Client, namespace string) (Backend, error) {
	client.Namespace = namespace
	client.Tags = append(defaultTagsFromEnv(), client.Tags...)
	return &datadogBackend{
		client: client,
	}, nil
}