prometheus/prometheus

Set and check (and document) a global label_value_length_limit

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#16525 opened on Apr 30, 2025

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

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

Tech stack
go
Domain
observabilitybackend
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 programmingPrometheus internalsLabel handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Review the existing label name length limit and label value length limit configuration options. Identify where label values are constructed during scraping and remote write handling. Consider the impact on label replace and label join. The assignee has been working on this; check their comments for specific file suggestions.