DatadogMeterRegistry: add support for publishing distributions

[monosoul]

Please describe the feature request.
At the moment DatadogMeterRegistry doesn't support publishing percentile histogram, while DD's API seems to support publishing distributions already: https://docs.datadoghq.com/api/latest/metrics/#submit-distribution-points .

Rationale
For DatadogMeterRegistry to reach feature parity with the statsd registry

[shakuzen]

Thanks for letting us know an HTTP API is available for sending distributions now. We previously abandoned efforts to support this with the Datadog registry due to the lack of API. I do still have concerns about how we could/should best handle such a feature. See my previous comment on it #1056 (comment). Unlike the statsd registry, the datadog registry is a step meter registry, which pushes metrics at step intervals. However, to send the raw data that datadog expects for distributions, we would need to either separately publish distribution data in near-realtime with recording or buffer the raw data for publishing. This would add significant complexity. It may still be worth adding if it provides enough value for users.

[monosoul]

Tbh I personally prefer using DatadogMeterRegistry to statsd registry because it has one less point of failure. If something doesn't work with statsd then we have to troubleshoot: the app itself, dogstatsd daemon, the channel between the app and the daemon and the channel between the daemon and DD's API. With DatadogMeterRegistry it's just the app, DD's API and the channel between them.

[shakuzen]

I realize we already had an open issue for this: #773. I'll close this as a duplicate of the older issue to keep things in one place.

[shakuzen]

Duplicate of #773