[incremental] expose duration since oldest pending read

As requested by go/incremental-disablement-metrics, we will expose the
duration since oldest pending read as part of the crash/ANR metrics.
This is the first step that exposes the value to Incremental Service.

BUG: 180951530
Test: unit test
Change-Id: Ic67460072556ef01780a1794b40924ca2092060d