feat: implement Prometheus metrics [WIP]

[dwisiswant0]

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Summary

Specs:

Metrics

• threat_datasets: Number of avaiable threat datasets. (Gauge)
• teler_event: Number of incoming teler WAF events. (Counter, labels: rule & threat)
• request_duration_seconds: Request duration times in seconds. (Histogram)

Note
Since there is no proposal for this, revisit inline TODOs and NOTEs that need to be reconsidered.

Proposed of changes

This PR fixes/implements the following bugs/features:

• Bug 1
• Bug 2
• Feature 1
• Feature 2
• Breaking changes

How has this been tested?

Proof:

Closing issues

Fixes #

Checklist:

• My code follows the code style of this project.
• My changes successfully ran and pass linters locally (run make lint).
• I have written new tests for my changes.
  • My changes successfully ran and pass tests locally.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4d628c2) 100.00% compared to head (9f2857c) 100.00%.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           36        36           
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  Hits            36        36           

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