Tutorials and examples for generating information-rich reports

[na--]

Prompted by this forum thread: https://community.k6.io/t/how-to-get-failure-statistics/1247

Whether these should be part of the docs or a tutorial on the blog is unclear, but we probably should have things like that:

• an example of generating an HTML report with the new handleSummary() function after Summary callback changes k6#1768
• and example script that parses our raw JSON and/or CSV metrics and generates a richer HTML or PDF report than the one above
• a grafana dashboard like the one in https://github.com/loadimpact/open-source-load-testing-stack#test-result-dashboard, but with InfluxDB

[MattDodsonEnglish]

We've updated the handleSummary docs with examples that I hope make how to use it and its available data more obvious.

We can also link to the blog post section of my "learning js through load testing".

It's far from amazing, but at least it shows a dynamic custom report. If someone wants to make a better example, we can, but I think most bases are covered.

https://k6.io/blog/learning-js-through-load-testing/#task-6-modify-k6-reporting

[ppcano]

@MattDodsonEnglish, consider this for the blog post about the different options to visualize k6 results.