feat: Add --profile option to lerna exec and lerna run

[bweggersen]

Adds an option to profile the run and exec commands. It produces a performance profile which can be analyzed using DevTools in a Chromium-based browser.

Description

Profiles the run and exec commands and produces a performance profile which can be analyzed using DevTools in a Chromium-based browser (direct url: devtools://devtools/bundled/devtools_app.html). The profile shows a timeline of the operations where each operation is assigned to an open slot. The number of slots is determined by the --concurrency option and the number of open slots is determined by --concurrency minus the number of ongoing operations. The end result is a visualization of the parallel execution of your scripts.

The default location of the performance profiles is at the root of your project.

Motivation and Context

We have over 200 packages in our repository and we see build time regressions from time to time. Visualizing lerna run build helps us uncover obvious mistakes or opportunities for improvement. For instance, in the example above, the final green box in thread 0 depends on the green box immediately preceding it on thread 1 (expanding the threads in the DevTool reveals the name of each package). By splitting the green package on thread 1, we can allow the final package on thread 0 to start much sooner, giving us a performance improvement of ~5 mins on all builds.

How Has This Been Tested?

I have tested this locally with all the argument permutations. If we should have some more formal testing of this feature, I would greatly appreciate some guidance on what that could look like.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[bweggersen]

Thank you for taking the time to review the PR!!

[evocateur]

Looks great, thanks!