core: remove `Collect` default impls for 0.2

[hawkw]

Feature Request

Crates

• tracing-core

Motivation

The Collect trait currently has several methods with default implementations that return an Option or some other "empty" or "default" type. In many cases, this isn't because it makes sense for those methods to be optional, but because they were added after v0.1.0 was released and making them mandatory would have been a breaking change. This can sometimes cause issues when implementations forget to override the default implementations and miss out on important behavior.

Proposal

For 0.2, we should consider making some of these methods mandatory so that implementations can't overlook them. In particular:

• Collect::max_level_hint probably shouldn't have a default None return value (and we should maybe get rid of the Option? is it actually worth differentiating between "may enable up to TRACE" and "just doesn't care about levels at all"?)
• Collect::current_span defaults to returning span::Current::unknown. Saying "I don't know what the current span is" should be an explicit decision on behalf of the collector in 0.2

[lenaschoenburg]

If we remove the default implementation of Collect::current_span we would need to make tracing_core::span::Current::unknown() public so that all implementers of Collect that relied on the default implementation have access to it.
This would also mean new imports of tracing_core in a bunch of places. Is this okay or should we re-export tracing_core::span::Current as tracing::span::Current?