WIP: Multi-tenant metrics server

[mostafa]

Ticket(s)

• Refactor metrics server to be multi-tenant #152

Description

The metrics server is multi-tenant (needs some integration work), yet the metrics merger feature seems to need more work to make it work as expected. Since a single instance of the plugin is always running in the entire lifecycle of a running GatewayD instance, it makes it hard to distinguish between metrics being emitted by the plugins. That is, a single (non-multi-tenant) metric is keeping track of the plugins' inner workings. This means that plugins should also support multi-tenancy.

Related PRs

N/A

Development Checklist

• I have added a descriptive title to this PR.
• I have squashed related commits together.
• I have rebased my branch on top of the latest main branch.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added docstring(s) and type annotations to my code.
• I have made corresponding changes to the documentation (docs).
• I have added tests for my changes.

Legal Checklist

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• I have read and understood the Code of Conduct.
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[mostafa]

This will be picked up later.