Include span in static directive

[pkoenig10]

Motivation

Using EnvFilter with a directive that specifies a span name but does not specify any field values requires using dynamic directives even though all the information necessary to match the directive is known statically from the metadata. This incurs unnecessary runtime costs.

Solution

This PR adds a span field to StaticDirective. Directives with span names but no field values are now considered static directives.

This PR intentionally does not change the FromStr implementation or the capabilities afforded by Targets (which uses this FromStr implementation for StaticDirective). StaticDirective is simply an implementation detail that happens to be used by both EnvFilter and Targets. This is admittedly somewhat confusing. It might be better to split StaticDirective into separate types - one used by EnvFilter and another used by Targets.

This PR also makes a minor clarification/standardization by renaming the in_span field to span. This matches the nomenclature used in the directive syntax in the docs (ie. target[span{field=value}]=level). I've also reordered the initializer fields to consistently match the declaration order.