Update documentation for http metrics

[ankur22]

We have renamed the browser metrics and changed them slightly. Here's what we have done:

1. Added a browser_ prefix to the http and data metrics. Rename metrics with a browser prefix xk6-browser#903
2. Removed majority of the http metrics which we think are most valuable to the user. Remove some of the http metrics xk6-browser#916
3. Removed the web vital rating metric (e.g. browser_web_vital_fid_good). Remove the web vital rate metric time series metrics xk6-browser#915

Here's what the summary output look like now:

     browser_data_received.......: 34 kB   9.6 kB/s
     browser_data_sent...........: 4.0 kB  1.1 kB/s
     browser_http_req_duration...: avg=105.84ms min=508µs   med=91.5ms  max=306.26ms p(90)=210.69ms p(95)=258.48ms
     browser_http_req_failed.....: 0.00%   ✓ 0        ✗ 9  
     browser_web_vital_cls.......: avg=0        min=0       med=0       max=0        p(90)=0        p(95)=0       
     browser_web_vital_fcp.......: avg=260.56ms min=124.7ms med=209.7ms max=447.29ms p(90)=399.77ms p(95)=423.53ms
     browser_web_vital_fid.......: avg=200µs    min=100µs   med=200µs   max=299.99µs p(90)=279.99µs p(95)=289.99µs
     browser_web_vital_inp.......: avg=24ms     min=24ms    med=24ms    max=24ms     p(90)=24ms     p(95)=24ms    
     browser_web_vital_lcp.......: avg=286ms    min=124.7ms med=286ms   max=447.29ms p(90)=415.03ms p(95)=431.16ms
     browser_web_vital_ttfb......: avg=195.36ms min=92.7ms  med=187.6ms max=305.79ms p(90)=282.15ms p(95)=293.97ms

We need to update the documentation and blog post that reference these metrics.

[ankur22]

This has been done as part of the larger change in commit.