Feature: expose /metrics for prometheus

[smeelock]

Context

We would like to monitor the number of drifting layers, with a metric monitoring tool e.g. Prometheus.

Feature requested

• exposed /metrics endpoint readable by prometheus
• burrito_layer_drift metric

here is an example "burrito_layer_drift" gauge metric created with a custom prometheus exporter for burrito

[corrieriluca]

We could use the controller-runtime to expose custom metrics: https://book.kubebuilder.io/reference/metrics