Add collect_timeout config option

[sudeephb]

Depends on https://github.com/canonical/prometheus-hardware-exporter/pull/67

This adds a collect_timeout option which sets the timeout for subprocess calls inside prometheus-hardware-exporter. Setting this value also changes the scrape_timeout scrape config option while relating grafana-agent with hardware-observer.

[Pjack]

Lesson learn: We should add new unit/func test when adding new configuration. Here is the defeat bug. https://github.com/canonical/hardware-observer-operator/issues/200 https://github.com/canonical/hardware-observer-operator/issues/207