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0090-remove-labelset-from-metrics-api.md

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Remove the LabelSet object from the metrics API

The proposal is to remove the current LabelSet API and change all the current APIs that accept LabelSet to accept directly the labels (list of key-values, or a map of key-values based on the language capabilities).

Motivation

The LabelSet API type was added to serve as a handle on a pre-defined set of labels for the Metrics API.

This API represents an optimization for the current metrics API that allows the implementations to avoid encoding and checking labels restrictions multiple times for the same set of lables. Usages and implementations of the metrics API have shown that LabelSet adds extra unnecessary complexity with little benefit.

Some users prefer to avoid this performance optimization for the benefit of a cleaner code and OpenTelemetry needs to address them as well, so this means that it is important for OpenTelemetry to support record APIs where users can pass directly the labels.

OpenTelementry can always add this optimization later (backwards compatible change) if we determine that it is very important to have.

Trade-offs and mitigations

In case where performance matters, here are the ways to achieve almost the same performance:

  • In the current API if a LabelSet is reused across multiple individual records across different instruments (one record to every instrument) then user can use the batch recording mechanism, so internally the SDK can do the labels encoding once.
  • In the current API if a LabelSet is used multiple times to record to the same instrument then user can use instrument bindings.
  • In the current API if a LabelSet is used across multiple batch recordings, and this pattern becomes very important, then OpenTelemetry can add support for batches to accept bindings.

To ensure that the current batch recording can help in scenarios where there are some local conditions that control which measurements to be recorded, the recommendation is to have the newBatchRecorder return an interface called BatchRecorder that can be used to add measurement and when all entries are added call record to record all the measurements.

Prior art and alternatives

Almost all the existing Metric libraries do not require users to create something like LabelSet when recording a value.