[CONFIGURATION] default sampler in config

[codeboten]

Steps to reproduce

Using the yaml configuration parsing, when a sampler is not specified in the yaml configuration, the sampler is not set on the tracer provider.

What is the expected behavior?

I expected the default sampler (parent based always on) to be set as defined in the configuration schema https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema-docs.md#tracerprovider

What is the actual behavior?

The sampler is null

[apatniv]

@codeboten I'd like to work on this issue if no one else is working on this.