DataDog Reporter to send EntityId from environment

[juancarrey]

Hello,

We are using latest Kamon library to send our metrics to DataDog though DataDog Agent (DogStatsD)

Our infrastructure relies on Datadog Entity Id to add contextual tags for infrastructure details, and we see that Kamon library is not adding this tag by default to DataDog reporters

dd.internal.entity_id:$DD_ENTITY_ID tag.

Essentially, the official java dogstatsd client adds this tag by default:

https://github.com/DataDog/java-dogstatsd-client/blob/c18559c89e24bd2e407a2c23dfd3608a831a2977/src/main/java/com/timgroup/statsd/NonBlockingStatsDClient.java#L1243

It would be good to add this tag by default as well in Kamon Datadog Reporters.

Thank you