Implement Metric Discovery Endpoints

## User Story

As a Backend.AI client developer, I want discovery APIs to find available metrics and presets so that I can build dynamic UIs without hardcoding metric names.

## Implementation Details

- Implement GraphQL query to list available metrics
- Implement GraphQL query to list available presets
- Implement REST endpoint for metric metadata
- Implement REST endpoint for preset catalog
- Include metric descriptions and units
- Include preset descriptions and required parameters
- Add filtering options for discovery queries
- Support versioning information in responses

## Acceptance Criteria

- Clients can retrieve list of all available metrics
- Clients can retrieve list of all available presets
- Metric metadata includes name, description, type, unit, labels
- Preset metadata includes name, description, category, parameters
- Discovery responses are cacheable
- API documentation includes discovery endpoint usage
- Integration tests verify discovery functionality

## Technical Notes

- Based on approved BEP specification
- Consider dynamic discovery from Prometheus metadata
- Cache discovery responses appropriately

JIRA Issue: BA-4055