TTree/RNTuple attribute to keep track of the loaded branches

[ikrommyd]

In Coffea, we would like a TTree/RNTuple attribute to keep track of the loaded branches similar to how we can track the number of requested bytes from a file:

uproot5/src/uproot/source/chunk.py

Lines 45 to 54 in b684d67

	@dataclasses.dataclass
	class SourcePerformanceCounters:
	"""Container for performance counters"""
	num_requested_bytes: int
	num_requests: int
	num_requested_chunks: int
	def asdict(self) -> dict[str, int]:
	return dataclasses.asdict(self)

This branch deserialization log should contain duplicates (i.e. if a branch is deserialized twice, it should be added twice to the log) and the log should be ordered in the way the branches were deserialized in time. However, I don't think it should contain duplicates if the branch was taken from the uproot cache. Overall, I think it should contain all the branches that uproot actually spent time deserializing in order. The simplest thing would be a list of strings I believe or you can make it a more complicated object to keep track of the number of bytes for example of each deserialization and other extra info you may want.

cc @pfackeldey @lgray

[ianna]

@ikrommyd - thanks for submitting the issue!