feat: add datadog meta headers

[DReigada]

Adds Datadog-Meta-* headers to the request that pushes the traces to the agent.

It seems that some specific tags are redacted by Datadog's backend and don't show in their UI if these headers are not set.
Two examples of these tags are: http.client_ip and network.client.ip.

We encountered this problem and contacted Datadog's support. This was there message with the solution:

Currently, all traces submitted with a client IP or similar value, have that client IP redacted on our back end automatically. To avoid this, you will need to set the Datadog-Meta-Lang and Datadog-Meta-Tracer-Version headers in the payload when making the requests to the Agent API. This should then let you keep the values anyway, and avoid them being redacted.

[GregMefford]

Thanks for figuring this out! 🚀