Add Datadog support

[rs]

@platinummonkey: I ported your Datadog source into the new code structure. The result does not work yet but and perhaps you could help me figuring why:

• Your implementation used an empty application key, which results into an "invalid application key" error.
• When both API and app keys are valid, the API returns empty series, I'm not sure why.

[platinummonkey]

Hey! Sorry, hadn't read my notifications in a while, the dd api client does do some validation on the input. As long as the API/APP Key have the correct lengths, it won't error out, but will also not return data as expected. I just hadn't gotten around to handling that case yet.

• If you're getting an empty series I suspect that the data may not be readily available on Dd itself? there is some delay between first metric+tag creation and the ability to query for it, though it's a minute or two. Did you have data in your demo account?

[rs]

Yes I have data in the account I'm testing on. The account contains production data.

[platinummonkey]

@rs can you post a sample query and drop me a ping on the public datadoghq slack channel? Should be able to help figure this out 👍