Prometheus: Backend refactor

[aocenas]

Over time the Prometheus backend changed a lot since initial migration. There were also some SDK changes that allow us to write some things in easier way. So few problems that should be solved:

• There are 2 different clients living in parallel (see Prometheus: Implement Streaming JSON Parser #48477)
• Duplicated HTTP setup code due to 2 clients. Prometheus: Use contextual middleware for req headers and simplify client creation #51061 Prometheus: Remove cache, pass headers in request, simplify client creation for resource calls and custom client #51436
• We handle custom headers manually instead of leaving that to the SDK and infra code. Prometheus: Remove middleware for custom headers #51518
• Additional HTTP client caching due to how custom headers are handled. Prometheus: Use contextual middleware for req headers and simplify client creation #51061 Prometheus: Remove cache, pass headers in request, simplify client creation for resource calls and custom client #51436
• Resource calls are trying to handle the response instead of just proxying the req/res. Prometheus: Remove cache, pass headers in request, simplify client creation for resource calls and custom client #51436

[szabalza]

@aocenas Please create a new ticket for tasks that will not be covered here and still have to be done

[aocenas]

Closing this and moving the remaining tasks to #52738