core(config): add category weight to perf config

[stevector]

I noticed that when running the Lighthouse with the --perf flag, the top level score value in the generated json report is 0 even when the score of the performance category is higher than 0. I expected these values to be the same when performance is the only category being evaluated.

This pull request simply adds default weights to performance, accessibility, and best-practices. pwa already had a weight.

Here's one way I tested this change.

git checkout master
/lighthouse-cli/index.js --chrome-flags="--headless --disable-gpu" https://www.github.com --perf  --output=json   --output-path=report.json  && cat report.json | jq '{TopLevelScore:  .score, PerfScore: .reportCategories[].score}'
{
  "TopLevelScore": 0,
  "PerfScore": 75.94117647058823
}

git checkout default-weight-value
./lighthouse-cli/index.js --chrome-flags="--headless --disable-gpu" https://www.github.com --perf  --output=json   --output-path=report.json  && cat report.json | jq '{TopLevelScore:  .score, PerfScore: .reportCategories[].score}'
{
  "TopLevelScore": 79.94117647058823,
  "PerfScore": 79.94117647058823
}

I encountered this problem when I was walking through https://github.com/ebidel/lighthouse-ci from @ebidel which relies on the top level score value of the json report generated by lighthouse.

[patrickhulce]

thanks for the contribution @stevector! good point about the --perf flag usage. For context, the PWA category was the only category with a score originally to maintain parity with Lighthouse 1.x that used the PWA audits as the toplevel score. It does make sense to have performance be the toplevel score of the perf config though.

How would you feel about just updating config/perf.json to have the performance category weight: 1 for now?

cc @vinamratasingal to potentially prioritize top-level score use cases and exploration later on

[stevector]

Thanks for the background info @patrickhulce! I hadn't thought about 1.x vs. 2.x as a factor. I'll push that change.

[patrickhulce]

awesome thanks for fixing this! 🎉 💯