feat: add Prometheus/Micrometer custom metrics instrumentation

[daimonbot]

Summary

Closes #4

Adds custom Micrometer metrics to the ldrbot Telegram bot for full observability. No new dependencies required — spring-boot-starter-actuator and micrometer-registry-prometheus were already present, and /actuator/prometheus was already exposed.

Approach

• Added BusinessMetricsConfiguration that registers and manages all custom meters
• Added GameSessionMetricsListener that listens to GameSessionRegistrationEvent and updates metrics
• Added MetricsConstants with metric name constants
• Updated GameSessionRegistrationEvent to include the necessary data for metric recording

Metrics Added

• ldrbot.game.sessions.total — counter, tagged by game type
• ldrbot.game.sessions.duration — timer/histogram of session duration, tagged by game type
• ldrbot.game.sessions.active — gauge of currently active sessions

[daimonbot]

PR ready for review: checks passing. Awaiting approval to merge.