Support Span Collection limit env variables

[codeboten]

As per the spec, we need to add support for Span Collection limits via environment variables:

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-environment-variables.md#span-collection-limits

[LetzNico]

Hey! I'd like to take on this one, looks pretty doable :)