[rush] allow Stopwatch to track multiple distinct intervals

[aramissennyeydd]

Summary

Discussed in #4680 - in some cases, a single Stopwatch that tracks a single time stamp isn't enough and you may want to track sub-spans. The specific use case would be to support timing the time to build and post to cache or restore from cache as separate trackable intervals.

Standard questions

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Question	Answer
@microsoft/rush globally installed version?
rushVersion from rush.json?
useWorkspaces from rush.json?
Operating system?
Would you consider contributing a PR?
Node.js version (node -v)?

[iclanton]

Sounds useful.