Rewritting the Metrics section

[agoncal]

Today our Metrics section uses MicroProfile Metrics. We should rewrite it with a more modern stack

File optional-observability/observability-metrics.adoc

[geoand]

cc @brunobat would you be able to rewrite this?

[brunobat]

I'd love to, will add it to my queue. Will use Micrometer.

[geoand]

Thanks!