Prometheus exporter add duplicated suffix for counter metrics

[ShadowySpirits]

Describe the bug
Prometheus exporter will add duplicated suffix _total.

Additional context
Prometheus exporter will add a _total suffix for all counter metrics. I think it should have a check that the metrics name already ends at _total.

opentelemetry-java/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/Serializer.java

Lines 515 to 520 in 3d5424a

	String headerName(String name, PrometheusType type) {
	if (type == PrometheusType.COUNTER) {
	return name + "_total";
	}
	return name;
	}

If the community agrees that this is a bug, I'm willing to submit a patch to fix it.

[jack-berg]

@dashpole this seems reasonable but I don't see any mention of this scenario in the prometheus openmetrics compatibility document. What do you think? Should the compatibility doc be updated to say that "_total" should be appended if not already present?

[ShadowySpirits]

@dashpole this seems reasonable but I don't see any mention of this scenario in the prometheus openmetrics compatibility document. What do you think? Should the compatibility doc be updated to say that "_total" should be appended if not already present?

Hi, @jack-berg. I notice that there is such a requirement in the specification:

Monotonic Sum metric points MUST have _total added as a suffix to the metric name.

[jack-berg]

Sorry @ShadowySpirits for the delayed reply. I see that the spec says "Monotonic Sum metric points MUST have _total added as a suffix to the metric name". I think for the behavior to be what you suggest the text would need to read something like: "Monotonic Sum metric points MUST have _total suffix. If the metric name does not end with _total, it MUST be added."

[ShadowySpirits]

Sorry @ShadowySpirits for the delayed reply. I see that the spec says "Monotonic Sum metric points MUST have _total added as a suffix to the metric name". I think for the behavior to be what you suggest the text would need to read something like: "Monotonic Sum metric points MUST have _total suffix. If the metric name does not end with _total, it MUST be added."

Your explanation makes sense, but repeating _total after metrics suffixed with _total is weird and causes different behavior from otlp-exporter. Is it possible to clarify the interpretation of the spec?

[jack-berg]

repeating _total after metrics suffixed with _total is weird and causes different behavior from otlp-exporter.

💯

@dashpole can you offer some insight here?

[ShadowySpirits]

@jack-berg @dashpole Have there been any updates? I noticed that my pull request was closed due to being stale.

[jack-berg]

Sorry about that - I thought we might get some feedback from the spec. Even though the spec doesn't explicitly describe the behavior of only adding "_total" if the name doesn't already end with it, I think its reasonable. I also think this is the behavior of the prometheus java http client. I believe this code strips the _total suffix if it exists, and this code appends it when serializing to the text format.

I re-opened and approved your PR. Thanks for you patience.

[ShadowySpirits]

@jack-berg Thanks a lot!

[dashpole]

Sorry, I was out on leave. Yes, please do not add a second _total if it is already the suffix of the metric. I can clarify that in the spec if you haven't already.

[jack-berg]

Thanks @dashpole. I did not update the spec.