stacklok.build_info exports as stacklok_build_info_ratio (drop WithUnit("1") upstream)

[glageju]

coremetrics.RegisterBuildInfo sets metric.WithUnit("1") (toolhive-core@v0.0.37/telemetry/metrics/buildinfo.go:50). The OTel Prometheus translator appends a unit suffix to a dimensionless gauge, so the metric exports as:

stacklok_build_info_ratio{commit="...",component="toolhive",version="..."} 1

Two problems:

1. _ratio is semantically wrong. build_info is an identity gauge that always observes 1; the identity lives entirely in its labels. It is not a ratio. Prometheus convention for this shape is a bare *_build_info (go_build_info, node_exporter_build_info).
2. The name is easy to get wrong in queries. A dashboard joining on stacklok_build_info returns empty. require.Contains(body, "stacklok_build_info") also passes on the suffixed name by substring, so a naive test asserts nothing — that bit us in Standardize proxy + vMCP metrics to stacklok.*, delete legacy twins #5956 (now anchored at line start).

Fix

Drop metric.WithUnit("1") in toolhive-core so the gauge exports unitless as stacklok_build_info, then bump the pin in toolhive and drop the (_ratio)? accommodation from the test regex and the docs note.

This cannot be fixed from the toolhive repo — RegisterBuildInfo lives in toolhive-core, so it needs a core release plus a pin bump here.

Interim

docs/observability.md documents the actual exported _ratio name so operators query something that exists. RFC §3.4 / AT #2 is satisfied either way (the gauge is present with component/version/commit); this is about the name customers will put in dashboards, so it's worth fixing before the vocabulary spreads.