Add micrometer.prometheus-proxy.connected-metrics-metadata property

[Mateo00]

Add micrometer.prometheus-proxy.connected-metrics-metadata property to enable or disable publishing Prometheus metrics metadata (lines "# HELP ..." and lines "# TYPE ...") in the payload returned by the /metrics/connected endpoint. The default value is True.

Disabling metrics metadata reduces the amount of data sent on each scrape.

In addition, some clients such as Telegarf expect the "# HELP ..." lines to be unique.
Currently, collecting metrics reported by the /metrics/connected endpoint fails with a Telegarf agent:
parsing metrics failed: reading text format failed: text format parsing error in line XX: second HELP line for metric name "XXXXX"

Disabling metrics metadata allows to work around this problem and scrapping the /metrics/connected endpoint with a Telegraf agent works well.

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[Mateo00]

Hi @shakuzen,
Is the PR compliant or is there a missing step on my end for it to be considered ?

[shakuzen]

I think I've understood the problem. Losing the HELP lines seems somewhat alright, but losing the TYPE lines seems potentially more problematic. Where are you eventually storing the metrics after Telegraf scrapes them, and is there no issue for you losing all type info?

[Mateo00]

After Telegraf scrapes, the metrics are stored in a VictoriaMetrics database. A Grafana connected to this database allows us to visualize the collected metrics.
In our tests, we didn't find any problems when the TYPE lines are deleted. All metrics are saved in the database and they display correctly in Grafana dashboards.

In the Prometheus documentation it's mentioned that HELP and TYPE lines are optional.
https://prometheus.io/docs/instrumenting/exposition_formats/#grouping-and-sorting :
Grouping and sorting
All lines for a given metric must be provided as one single group, with the optional HELP and TYPE lines first (in no particular order). Beyond that, reproducible sorting in repeated expositions is preferred but not required, i.e. do not sort if the computational cost is prohibitive.
Each line must have a unique combination of a metric name and labels. Otherwise, the ingestion behavior is undefined.