Make Sample configurable

[Sebruck]

Problem
Currently the Sampler for each span is fixed to Sampler.alwaysSample(), which is not feasible for most of the production setups.

Goal
Make it possible to configure the sampler.

Ideas
Via configuration file: Percentage spans sampled (as default)
Via code: When creating a span or using one of the trace functions it should be possible to override the sampler individually

[Sebruck]

Partial done. A global default can be configured in the config.

Sebruck@4179154

Missing: Override default on span basis