Replace Valkey-based Live Stats with Prometheus-backed Implementation

[seedspirit]

Background

The current live statistics system uses a custom implementation where:

• Agents collect metrics via StatContext and store them in Valkey (Redis) with
  short TTLs (8-120 seconds)
• Manager serves these stats through GraphQL live_stat fields (kernel.live_stat,
  agent.live_stat)
• Data is serialized with msgpack and queried via ValkeyStatClient

Meanwhile, a separate Prometheus integration already exists(ContainerMetricService) that queries metrics from Prometheus but is not used for the live_stat API.

Problem

• Duplicate data paths: Metrics are sent to both Valkey (for live stats) and
  Prometheus (for monitoring/dashboards)
• No historical data: Valkey-based stats are ephemeral with short TTLs, making
  trend analysis impossible
• Maintenance burden: Two separate metric pipelines to maintain
• Inconsistency risk: Valkey stats and Prometheus metrics may diverge

Proposed Solution

Replace the Valkey-based live stats system with a Prometheus-backed implementation:

1. Agent side: Remove Valkey stat publishing; ensure metrics are exported to
   Prometheus
2. Manager side: Modify live_stat GraphQL resolvers to query Prometheus via
   ContainerMetricService instead of ValkeyStatClient
3. Deprecate: Phase out ValkeyStatClient for statistics (keep ValkeyLiveClient for
   service discovery)

Benefits

• Single source of truth: All metrics flow through Prometheus
• Historical queries: Access to time-series data for trends and analysis
• Ecosystem integration: Native compatibility with Grafana, alerting, etc.
• Reduced complexity: Eliminate Valkey stat storage and TTL management
• Lower Valkey load: Remove high-frequency stat read/write operations

JIRA Issue: BA-4039