Documenting example labels

[lambdanis]

metricsmd tool iterates over a Prometheus metrics registry and documents all metrics with their label values. This works well if all possible label values are known beforehand.

However, observability tools like Tetragon expose metrics with labels depending on the environment, e.g. Kubernetes namespace. Such metrics can't be initialized with all possible label values.

How Tetragon deals with it is by initializing metrics for docs with example label values. See for example here. This works, however, the generated docs don't distinguish between the real label values and the example ones. While in most cases it's pretty obvious which values are examples, it would be nice if metricsmd supported rendering them differently.