Integrate with datadog API

[jrask]

When running as AWS Lambda, there is no running statsd or datadog agent, but we would like to be notified about how things are going.

By implementing datadogs http api this could be really simple. Not-really-thought-through-preudo-code

.doOnNext (
     datadog.increment (
         api_key: 'key',
         metric: 'lumbermill.success'    
    )
)
.doOnError (
     datadog.increment (
         api_key: 'key',
         metric: 'lumbermill.error'    
    )
)

We could also handle specific values from contents

.doOnNext (
     datadog.increment (
         api_key: 'key',
         metric: 'api.statuscode.{backend_status_code}'    
    )
)

[jrask]

Will add this again when suitable.