Write BEP for Prometheus Client layer seperation

[seedspirit]

Background

Currently, ContainerUtilizationMetricService mixes multiple concerns:

• Direct aiohttp HTTP calls (no connection pooling)
• PromQL query string generation
• Metric type detection (gauge/rate)
• Business logic coordination

This tight coupling makes it difficult to:

• Extend to other metric types (agent, node metrics)
• Test query building logic in isolation
• Reuse Prometheus access from other services

Proposed Solution

1. Extract PrometheusClient

• Create clients/prometheus/ package with dedicated client
• Use ClientPool for connection reuse
• Add proper error handling (PrometheusQueryError, PrometheusTimeoutError)

1. Introduce Querier Interface Hierarchy
   PrometheusMetricQuerier (base: name, labels)
   └── AggregatingMetricQuerier (adds: group_by_labels)
   └── UtilizationMetricQuerier (adds: query_strategy)
2. Business-Oriented Client API

• query_utilization(querier, time_range) - accepts abstract querier type
• fetch_available_container_metrics() - metadata retrieval
• Query building logic moved to client as private methods

Acceptance Criteria

• PrometheusClient extracted with ClientPool integration
• Querier interface hierarchy defined (PrometheusMetricQuerier → AggregatingMetricQuerier → UtilizationMetricQuerier)
• ContainerMetricQuerier implements query_strategy() for automatic gauge/rate selection
• ContainerUtilizationMetricService simplified to thin coordination layer
• All existing tests pass with updated mocks
• New tests for query_strategy() method

JIRA Issue: BA-4229