Datadog API Reporter fails to authenticate

[thereisnospoon]

I'm getting 403 response from Datadog API when trying to use API reporter as described in https://kamon.io/docs/latest/reporters/datadog/#api-reporter
I've checked the reporter code and it looks the way api-key is being passed is probably outdated: https://github.com/kamon-io/Kamon/blob/master/reporters/kamon-datadog/src/main/scala/kamon/datadog/package.scala#L69
The current implementation passes the api-key as an api_key query parameter, while according to the Datadog docs the api-key should be passed using DD-API-KEY header.