Expose JVM metrics with Prometheus inline exporter about mulog

[BrunoBonacci]

Currently the μ/log Prometheus publisher converts events into metrics by creating a new metrics for every numerical value.
It uses string values as labels. However, it only looks at numerical values on the top-level event map, it doesn't nest.

See here:
https://github.com/brunobonacci/mulog/blob/master/mulog-prometheus/src/com/brunobonacci/mulog/publishers/prometheus/metrics.clj#L70

One option/workaround is to add a :transform function on your
Prometheus publisher to copy the metrics you are interested into
publishing on the top level.

If you want all metrics you will need to "flatten" a nested map while
preserving the names.

I will think whether this could be something that it is done
automatically by μ/log publisher. My worry is that this might cause an
explosion of metrics to be published.

[BrunoBonacci]

Reopening as this issue needs more work.

[BrunoBonacci]

This issue if fixed (thanks @piotr-yuxuan) it will be released with the next release (v0.7.2 or v0.8.0)