More configurable span tracing categories

[nagisa]

Today this library registers a single tracer which in turn unconditionally registers all the hooks it is interested at once:

tracing-gstreamer/src/tracer.rs

Lines 108 to 117 in 9b7d80b

	self.register_hook(TracerHook::PadPushPost);
	self.register_hook(TracerHook::PadPushPre);
	self.register_hook(TracerHook::PadPushListPost);
	self.register_hook(TracerHook::PadPushListPre);
	self.register_hook(TracerHook::PadQueryPost);
	self.register_hook(TracerHook::PadQueryPre);
	self.register_hook(TracerHook::PadPushEventPost);
	self.register_hook(TracerHook::PadPushEventPre);
	self.register_hook(TracerHook::PadPullRangePost);
	self.register_hook(TracerHook::PadPullRangePre);

If we wanted to extend the scope of the tracing this library does (which I would very much like to,) we sort-of need to figure out a way to make the trace interest more configurable. One option would be to actually have different Tracer objects for each different trace categories. We could alternatively expose the Tracer as a public API and handle the registration of different events outside of the object creation, giving users granular control to do the same for themselves.

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If nothing else, this gives a great way for people to mitigate locally issues that might occur as a result of bug in bindings or this library in tracers for a specific event.