fix: don't escape UTF-8 names by default for OpenMetrics 2.0 - #2519
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October 4, 2026 00:10
OpenMetrics 2.0 supports UTF-8 names and has no escaping parameter, but PrometheusScrapeHandler picked the escaping scheme before the writer and fell back to underscores whenever the Accept header had no escaping term. Prometheus does not send one for OM 2.0, so UTF-8 names were escaped. Choose the escaping scheme after findWriter and default to allow-utf-8 when the OM2 writer serves an explicit version=2.0.0 request. An explicit escaping term still wins, and OM 1.0, unversioned OpenMetrics and text requests keep the underscores default, also with contentNegotiation=false. EscapingScheme.fromAcceptHeader gets an overload taking the fallback scheme. Signed-off-by: KR Ravindra <42912207+KR-Ravindra@users.noreply.github.com>
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Fixes #2517
Problem
A scrape with
Accept: application/openmetrics-text;version=2.0.0and noescapingparameter still gets UTF-8 names escaped to underscores.PrometheusScrapeHandlerpicks the escaping scheme beforefindWriterand falls back tounderscoreswhenever the header has noescapingterm. OpenMetrics 2.0 has no such parameter and Prometheus doesn't send one.Fix
findWriter. If the OM2 writer serves a request that explicitly asked forversion=2.0.0, the fallback isallow-utf-8; otherwise it staysunderscores.contentNegotiation=false, where the OM2 writer also serves OM 1.0 and unversioned requests from scrapers that may not handle UTF-8 names.escapingterm still wins.EscapingScheme.fromAcceptHeader(String, EscapingScheme fallback); the existing method delegates withDEFAULT.How tested
New
PrometheusScrapeHandlerTestcases:version=2.0.0keepsmy.counter(fails on main),version=2.0.0;escaping=underscoresescapes, and OM 1.0, unversioned OpenMetrics andtext/plainkeep underscores, each with content negotiation on and off. PlusEscapingSchemeTestfor the new overload.Prepared with an AI agent operated by KR-Ravindra, who reviewed and tested it. The agent is pointed only at issues maintainers have confirmed and problems he has hit, not at producing contributions.