docs: document how to write create new keptnmetricsprovider

[geoffrey1330]

Fixes #1248

Description

This PR documents the process for creating a new KeptnMetricsProvider within Keptn. It provides detailed instructions on defining a new provider type, implementing the provider, and instantiating it within the Keptn environment. Additionally, it covers the necessary updates to validation webhooks, CRD configs, and Helm charts to ensure seamless integration of the new provider.

Checks

• My PR fulfills the Definition of Done of the corresponding issue and not more (or parts if the issue is separated
  into multiple PRs)
• I used descriptive commit messages to help reviewers understand my thought process
• I signed off all my commits according to the Developer Certificate of Origin (DCO)(
  see Contribution Guide)
• My PR title is formatted according to the semantic PR conventions described in
  the Contribution Guide
• My content follows the style guidelines of this project (YAMLLint, markdown-lint)
• I regenerated the auto-generated docs for Helm and the CRD documentation (if applicable)
• I have performed a self-review of my content including grammar and typo errors and also checked the rendered page
  from the Netlify deploy preview
• My changes result in all-green PR checks (first-time contributors need to ask a maintainer to approve their test runs)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ce57993) 85.75% compared to head (1c918d9) 86.77%.
Report is 57 commits behind head on main.

❗ Current head 1c918d9 differs from pull request most recent head 5843060. Consider uploading reports for the commit 5843060 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2939      +/-   ##
==========================================
+ Coverage   85.75%   86.77%   +1.02%     
==========================================
  Files         162      162              
  Lines       10351     8514    -1837     
==========================================
- Hits         8876     7388    -1488     
+ Misses       1186      837     -349     
  Partials      289      289              

see 139 files with indirect coverage changes

Flag	Coverage Δ
certificate-operator	69.23% <ø> (+1.80%)	⬆️
component-tests	57.13% <ø> (+1.13%)	⬆️
lifecycle-operator	86.39% <ø> (+1.10%)	⬆️
metrics-operator	88.32% <ø> (+0.68%)	⬆️
scheduler	34.74% <ø> (-1.79%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[StackScribe]

Nice job! Thanks for taking this on -- it's an important piece of doc that we really need!

Any idea why the doc file is not showing in the readthedocs preview?

[geoffrey1330]

Nice job! Thanks for taking this on -- it's an important piece of doc that we really need!

Any idea why the doc file is not showing in the readthedocs preview?

No idea yet but i will look into it.

[RealAnna]

@geoffrey1330 this is great! Could you please rebase and fix the conflicts? I think we are good to merge

[geoffrey1330]

@geoffrey1330 this is great! Could you please rebase and fix the conflicts? I think we are good to merge

Done

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud