[AF05] Fill NetworkStatsRecorderFileReading metrics

By verifying with benchmark tests, this does not have significant
change on file reading performance.

Test: atest ConnectivityBenchmarkTests
Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.NetworkStatsRecorderTest
Bug: 313841499
Change-Id: I161a101dcec82fdaa70128541f9e153bac484ecc