Add datadog telemetry

[jippi]

Closes #1815

This is my first go-code ever, so please be kind :)

[jippi]

@dadgar not sure what I'm missing here, but adding

telemetry {
  datadog_address = "localhost:8125"
}

to my config yields telemetry -> invalid key: datadog_address

[diptanu]

@jippi You need to add datadog_address in Valid keys here - https://github.com/hashicorp/nomad/blob/master/command/agent/config_parse.go#L526

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.