enhancement: add a new DogStatsD-specific exporter

[tobz]

Context

This PR adds a brand new exporter, metrics-exporter-dogstatsd, which is specifically designed for sending metrics using the DogStatsD protocol, typically to a Datadog Agent or DogStatsD server.

While metrics-exporter-statsd exists and works well for many use cases, it depends on cadence which ultimately limits the maximum performance and capabilities of any exporter depending on it. This shows up in terms of cached access to metrics and things like optimal payload building, and so on.

Further, while libraries like cadence do strive to support DogStatsD-specific features, it doesn't aspire to (to my knowledge) match support with the official DogStatsD client libraries, while metrics-exporter-dogstatsd does. This includes support for client-side aggregation, histogram sampling, smart reporting, and more.

Notes

We've done some unrelated refactoring/cleanup in metrics-util to add the necessary bits for the new exporter.