Exporter to store metrics in TimescaleDB (PostgreSQL).

Prerequisites

• Resolve Update GORM to v2 #42
• Resolve Update check ID from uint to string #45

Steps

• Remove PrepareChecks from Exporter interface.
• Add GetMetrics(context.Context, time.Duration, checkIDs ...string) method to Exporter interface.
• Update existing exporter ("log").
• Add metrics model and implement exporter.