Title: Add Dynatrace custom sampler

Description:

Dynatrace has been contributing to Envoy to enhance the OpenTelemetry support. Envoy now has support to export spans via HTTP, can enrich the spans with resource attributes and configure a custom sampler. The only "missing" feature for Dynatrace customers to be able to migrate to a "full OTel solution" in Envoy is a Dynatrace Sampler.

In a nutshell, Dynatrace customers can configure the sampling rate in which spans are sampled, thus achieving a dynamic sampling behavior. This allows customers to effectively cost-control by limiting the amount of spans exported without losing important information about their environments.

Thus Dynatrace would like to propose and add a custom sampler to Envoy that is aligned with the our current sampling features, so users using Envoy in a environment where Dynatrace is present, they can have consistent and correct sampling throughout their applications.