subscriber: apply envfilter's target filter to events as well as spans

[jvimal-eg]

Motivation

Following a conversation on Discord, I learnt that an EnvFilter
constructed like this:

target[span{field=value}]=debug

enables the debug level for any event emitted within the context of
span.

It would be ideal if the debug level were consistently applied to
events as well as spans. This would mean that the target=debug filter
would also be applied to events generated within the context of the
span.

Solution

Add a new field inside CallsiteMatcher to keep track of the most
specific target match among Directives that match a callsite Metadata.

Open questions:

• It looks like a clone of a String is needed. Unsure if there is a
  way to avoid it?
• We take the maximum level that matches a callsite. It's unclear what
  we should do in cases where there's a conflict between the maximum
  level and the most specific target match:

target[span{field=value}]=debug,target::foo[span{field=value}]=info