[metrics] add namespace, subsystem and refactor names

[axelfahy]

Proposed changes

• Add namespace and subsystem for prometheus metrics
• Rename metrics according to prometheus guidelines

Related MR for docs modification: OpenCTI-Platform/docs#286

Related issues

• N/A

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

[marieflorescontact]

Hi @axelfahy ,
than you for your contribution!
Could you open a documentation PR, as this could bring breaking changes for some of our users? => https://github.com/OpenCTI-Platform/docs

[marieflorescontact]

thanks for your contribution 🙂
it will be in the next major release