prometheus/prometheus

Set and check (and document) a global label_value_length_limit

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#16,525 opened on Apr 30, 2025

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component/documentationcomponent/scrapinggood first issuehelp wantedkind/featurepriority/P3

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Description

Proposal

Prometheus has label_name_length_limit and label_value_length_limit, currently defaulted to no limit.

After #16069, Prometheus will crash if you give it a name or value longer than 16MB, so we should at least have a default limit lower than that.

I think that a reasonable limit would be more like 200 bytes, but I am aware of people doing exciting things like storing a bitmap in a label. Maybe we could go with something like 1MB?

The code checking limits while scraping would need to check before constructing the Labels data structure, and we should check in other places such as the remote-write receiver, label_replace, label_join.

Contributor guide

Research direction
Investigate the existing label name length limit implementation, determine safe default limit values, and ensure limits are enforced in all ingestion paths.
Tech stack
go
Domain
backend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
Go
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60