[ntuple] add several useful performance metrics

[jblomer]

Explain what you would like to see improved and how.

There has been a proposal to include the following useful metrics:

1. Sparseness: Total bytes read divided by the total file size.
2. Randomness: Sum of the skip distances, divided by the total bytes read.
3. Transactions: Number of interactions with the I/O layer per file.

Moved from https://its.cern.ch/jira/browse/ROOT-5180

ROOT version

master

Installation method

from sources

Operating system

n/a

Additional context

No response

[003-kalki]

If no one else is working on it, could you please assign this issue to me?
@jblomer