fix pool agent metric value

[gamoutatsumi]

Change a pool-gent metric values to enumerated container status for tracing container status in time series.

Summary (Generated)

This pull request to pool-agent includes changes to improve the collection and representation of LXD instance metrics. The most important changes include modifying the way instance statuses are handled and displayed, and updating the associated Prometheus metrics.

Improvements to metrics collection:

• pool-agent/cmd/agent.go: Changed lxdInstances.WithLabelValues to set the status as a float value using containerStateFromString instead of incrementing a counter.

Enhancements to metrics representation:

• pool-agent/cmd/metrics.go: Added a containerState type and constants to represent different container states numerically. Implemented String and containerStateFromString methods to convert between string and numeric representations of container states.
• pool-agent/cmd/metrics.go: Updated the lxdInstances help description to include the enumeration of container states. Removed the status label from the lxdInstances metric.