Add an OpenTelemetry handler

[Pryz]

This PR adds a first version of an OpenTelemetry handler. This handler implement the otlp protocol with an HTTP Client.

There are few caveats with the current implementation:

• It only supports HTTP but GRPC should be easy enough to add later if needed.
• The client is written in a fire-and-forget fashion, meaning that metrics will be sent once even if there is a failure when sending the request.
• The handler only support backends which expect cumulative values such as Prometheus.

[Pryz]

Thanks for the review @kevinburkesegment !