Implement Stellar Provider Latency Observatory

[mijinummi]

🧠 Concept

Track latency metrics across integrated providers.

⚠️ Problem

Provider responsiveness varies over time.

📁 Implementation Scope

• src/observability/latency/providers/stellar/

✅ Requirements

• Measure provider latency
• Generate latency reports
• Detect degraded performance

🎯 Acceptance Criteria

• Latency observatory implemented
• Latency metrics available