A way to specify metric aggregator

[morigs]

I cannot believe this functionality is still missing but also I cannot find any related issues/PRs. Feel free to close if it's a duplicate.

I am a library author. I need to expose a "latency" metric with ability to discover its avg and max values in the future. According to the otel spec I have chosen ValueRecorder:

const latencyRecorder = meter.createValueRecorder('processing_latency', { description: 'Processing latency.' });

As we can see here the default aggregator for ValueRecorder is MinMaxLastSumCountAggregator. It seems MinMaxLastSumCountAggregator is suits me. Unfortunately MinMaxLastSumCountAggregator metric is exported as a single value...
I want to change aggregator to histogram (and use Prometheus histogram functions for analytics).
Unfortunately there is no way to specify aggregator during metric creation. Batcher is always uses its default. And as a library author I have no control over batcher and exporter.
Consider add an option to the metric constructor that allows to pass an aggregator.

(And offtopic: why ValueRecorder metric is not exported as summary? Am I doing something wrong? I use exporter-collector. And collector uses prometheusexporter)

[dyladan]

I cannot believe this functionality is still missing

Please refrain from using judgmental language

The metrics sdk is currently very beta and is known to be behind the spec. We've been focusing effort on tracing recently as tracing spec moves closer to GA.

@obecny is the maintainer with the best knowledge of metrics right now. @obecny are you aware of a way to set an aggregator?

[vmarchaud]

You can specify it by using a custom batcher, there is a doc here: https://github.com/open-telemetry/opentelemetry-js/blob/master/doc/batcher-api.md
I would suggest to read about the current situation (mainly about the story around Metrics Views) before making judgement.

[morigs]

Story for this misunderstanding. I just said what I was thinking. IMHO this is a must have feature and I'm ready to contribute if we will reach consensus on API and implementation details

[morigs]

@vmarchaud I trying to figure out the way to do this.
I don't judge, I was trying to said "I'm not sure is this feature really missing or I don't know about it"

In fact as a library developer I have no control over Batcher. I can only create and update metrics. Am I wrong?

[obecny]

MinMaxLastSumCountAggregator will be removed soon and recorder will get MinMaxSumCount and observer will get LastValue only. You can create your own batcher and then do the aggregation to whatever you need as well (@vmarchaud mentioned this already) so you are not limited to any aggregator if you don't want to. The metrics as mentioned by @dyladan still lacks of few things due to spec changes and some of the part is not finished yet (talking about spec so hard to have everything earlier :)).

[morigs]

Thanks for answers. If I understand correctly views API is what I was looking for.

May be consider creating a tracking issue? This will allow to submit a PR with draft implementation as done in the python library
open-telemetry/opentelemetry-python#596

[morigs]

So what is the recommended way to change the aggregator at the moment if a batcher is not available?

[AndrewGrachov]

I use somthing like this. It's kinda hack, but, as mentioned above, metrics are in infancy

type GetBoundaries = (name: string) => number[];

export class CustomHistogramBatcher extends UngroupedBatcher {
	private getBoundariesFor: GetBoundaries;

	constructor(getboundaries: GetBoundaries) {
		super();
		this.getBoundariesFor = getboundaries;
	}

	aggregatorFor(descriptor: MetricDescriptor): Aggregator {
		if (/_histogram$/.test(descriptor.name)) {
			const boundaries = this.getBoundariesFor(descriptor.name);
			return new HistogramAggregator(boundaries);
		}
		return super.aggregatorFor(descriptor);
	}
}

		this.meter = new MeterProvider({
			exporter,
			interval: this.interval,
			batcher: new CustomHistogramBatcher(this.getHistogramBoundariesFor),
		}).getMeter(this.serviceName);

[AndrewGrachov]

Would be nice to specify aggergator per measure, but IDK what standard will say

[vmarchaud]

@AndrewGrachov Thats pretty much the goal of the views api which is discussed in the oteps repo.

So what is the recommended way to change the aggregator at the moment if a batcher is not available?

@morigs Not sure to follow, when is a batcher not available ? There are no other choice right now.

[morigs]

Ok thank you. I think this question may be closed now.
But please consider creating tracking issue for views api implementation