feature: Add prometheus demo

[LXPWing]

What this PR does:
Add prometheus demo

Fixes #466

[codecov]

Codecov Report

Merging #480 (d5226ec) into main (1947177) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #480   +/-   ##
=======================================
  Coverage   60.70%   60.70%           
=======================================
  Files         120      120           
  Lines        6382     6382           
=======================================
  Hits         3874     3874           
  Misses       2139     2139           
  Partials      369      369           

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[seeflood]

There are some misunderstandings here 😢
In fact, I mean adding a quickstart document, including some steps like deploying prometheus and layotto, running a demo, and opening the web page to see the effect.
You can take the skywalking quickstart demo as an example

[seeflood]

@LXPWing Hi, the new CI is merged and now we can continue with this PR :)

[seeflood]

@LXPWing hi, I modified your branch. I deleted prometheus.png and upload it to a CDN, so that readers can download this image faster.
Please take care not to git push -f and overwrite it :)

[LXPWing]

The existing error has been fixed, thank you for your review.

[LXPWing]

I don't know how to solve this problem QaQ

[seeflood]

@LXPWing It seems that github workflow could not connect to codecov successfully. Let's try again

[seeflood]

LGTM.
I deleted docs/img/trace/layotto.png and uploaded it to the CDN.
Thanks for your contribution!