add possibility to choose sampling strategy

[cawolf]

Despite having no consensus on a specification for sampling in general, most tracers are actually providing two basic tracer types:

• constant samplers (always tracing, never tracing)
• probabilistic samplers

Right now, we are enabling the constant samplers (always tracing) by default for tracing implementations. However, as high-traffic projects are going to use this bundle, there will be the need of choosing other strategies (probably at configure-time, maybe even at runtime).

In order to provide this feature to bundle users, we should research how current tracer implementations are handling and passing these configurations and maybe come up with an abstraction.

[cawolf]

Upon further discussion with @mmross we came to the conclusion, that right now we want to configure the sampling behaviour individually per tracer implementation. Coincidently, both Jaeger and Zipkin provide implementations for Const and Probabilistic sampling, but this is currently not enough for a general abstraction. We will, however, name both vendor configurations identically, so that a future abstraction can be done smoothly.

[cawolf]

All PRs are merged, closing