Convert summary result as Prometheus Metrics

[mhoshi-vm]

We'd like to see the vulnerability reports as a metric.
Similar ideas from here.
https://github.com/devopstales/trivy-operator#usage

[chen-keinan]

@mhoshi-vm are you familiar with the starboard-exporter project ?
it produces metrics from VulnerabilityReport, ConfigAuditReport CRDs , trivy-operator CRDs are similar to starboard CRDs.
looking forward it on our plans to adjust the metrics to be more trivy oriented #78