metrics: replace time.Tick with time.NewTicker to avoid leaked timers

[anim001k]

Summary

Replace time.Tick with time.NewTicker in the OpenTSDB exporter to enable explicit stop and prevent leaked timers.

Rationale

time.Tick cannot be stopped and may leak timers/goroutines over time. time.NewTicker allows proper lifecycle management and graceful shutdown.

Changes

• metrics/opentsdb.go: use time.NewTicker(c.FlushInterval) with defer ticker.Stop().

Impact

• Improves resource management and shutdown behavior.

[jwasinger]

The function never exits, so I don't see how this improves anything.