TLDR: increase expected upper bound for idxEntryBytesWritten or remove check altogether.

In our CI system, index_build_operation_metrics.js has failed occasionally due to an unexpected idxEntryBytesWritten value. Debug log messages enabled in SERVER-83825 showed that there were write conflicts in updating the config.system.indexBuilds collection:

[js_test:index_build_operation_metrics] d21540| 2024-03-08T00:43:23.067+00:00 D1 WRITE    4640401     [conn20] "Caught WriteConflictException","attr":{"operation":"removeIndexBuildEntry","reason":"Caused by ::  :: Please retry your operation or multi-document transaction.","namespace":"config.system.indexBuilds","attempts":0}

which would give us a higher than expected value for the idxEntryBytesWritten metric. Write conflicts are an expected part of a normal server operation and this should be reflected in the test expectations.

In addition to revisiting the idxEntryBytesWritten check, we should also update the check on idxEntryUnitsWritten