[Prometheus SPECS] Only one HELP line may exist for any given metric name; Prometheus doesn’t like one metric having different help strings

[incertum]

https://prometheus.io/docs/instrumenting/writing_clientlibs/

"Gauge/Counter/Summary/Histogram MUST require metric descriptions/help to be provided."

https://prometheus.io/docs/instrumenting/exposition_formats/

"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)."

"Only one HELP line may exist for any given metric name."

https://prometheus.io/docs/instrumenting/writing_exporters/

"Prometheus doesn’t like one metric having different help strings."

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While not conforming to any of this doesn't break Prometheus, new guidance shall be to enforce the same HELP line per metric name / MetricFamily and only emit it once without configurability to do otherwise.

However, to maintain backward compatibility, enforcing it is still up for debate. It is stated that "Gauge/Counter/Summary/Histogram MUST require metric descriptions/help to be provided." At the same time, Prometheus works just fine without providing a HELP line, and at a different place in the Prometheus docs, TYPE and HELP are explicitly stated as optional. Note that we would break backward compatibility if we were to enforce a HELP line.