subscriber: add set_span_events
to fmt::Subscriber
#2962
Merged
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Motivation
Using reload::Layer with a Filtered layer makes it possible to activate different tracing levels for various modules dynamically. However, it's not currently possible to modify the currently span event configuration since with_span_events can only be set when creating the layer. Due to #1629 it's not possible to completely re-load a layer and so span config is essentially set-once.
Solution
This PR adds a method similar to
set_ansi
to allow modifying the fmt span configuration in a call toreload::Handle::modify
Closes #2957