Align sampling specs in SDK

[lzchen]

Description

Part of #906 to adhere to https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#sampling
Updates classes to adhere to the sampling spec.

Fixes # (issue)

Sampling changes

• Decision is an enum.
• Change Decision -> Sampling Result
• should_sample returns sampling result
• Remove span_id from should_sample fn def
• Seperates ProbabilitySampler from ParentOrElse
• ParentOrElseSampler new class
• GetDescription method for all samplers
• change is_recording_Events to is_recording
• Remove "Optional" typing for Attributes in some method definitions (Attributes already has Optional typing)

TODO

1. Make attributes in samplers immutable
2. how to handle is_recording -> send to processor/exporter

[codeboten]

Most of the changes here look good, couple of minor suggestions/questions.

Should ParentOrElse be ParentBased? Looking at the spec, it looks like it was renamed: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#parentbased

[aabmass]

Should ParentOrElse be ParentBased? Looking at the spec, it looks like it was renamed: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#parentbased

Also looks like ProbabilitySampler should be TraceIdRatioBased?

[codeboten]

Change looks good, thanks for the update!