We would like visibility into the duration of each index build phase. To do this,

1. Add metrics to indexBuildsSSS for each duration.
2. Calculate elapsed time for each phase and store the value.

It's convenient that we already have counters for each phase, and each counter is incremented in a function call where we have access to ReplIndexBuildState, so we can just calculate the elapsed time at each location where the respective counter is incremented using ReplIndexBuildState::getIndexBuildMetrics() to get startTime.