Metrics reporting for scan and commit

This Issue tracks the implementation of metrics reporting for scan and commit operations.
design doc: https://docs.google.com/document/d/1oF1LWN3_-wXf831nvOngHzc5_3WDh-iB-LEMnOYokpA/edit?tab=t.0

Start with the linked design document, then locate the scan and commit entry points in the C++ repository. Use the design document to determine which metrics must be reported and where they belong. Done means scan and commit operations expose the metrics specified by the design, with any required tests or validation identified during implementation.

Written by the indexing model from the issue text.