[Docs] Implementing Agent Metadata Service

[Future-Outlier]

Tracking issue

#3936

Why are the changes needed?

Make users implement their own agent metadata service.

What changes were proposed in this pull request?

Add Implementing Agent Metadata Service documentation.

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.01%. Comparing base (d68047a) to head (a0c675a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5109      +/-   ##
==========================================
+ Coverage   58.56%   59.01%   +0.45%     
==========================================
  Files         494      645     +151     
  Lines       42987    55670   +12683     
==========================================
+ Hits        25176    32855    +7679     
- Misses      15816    20221    +4405     
- Partials     1995     2594     +599     

Flag	Coverage Δ
unittests	59.01% <ø> (+0.45%)	⬆️

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[neverett]

Would you be able to add these changes to #5106 so all changes to the agents docs can be reviewed together? Thanks!