Data race when using promhttp handlers without methodor code but with WithLabelFromCtx

[tigrato]

When using any promhttp handler or roundtripper instrumentation middleware, we have a data race if the metric doesn't record code nor method but uses WithLabelFromCtx.

It exists because when we don't record code or method, the labels function returns a pre-created map called emptyLabels.

client_golang/prometheus/promhttp/instrument_server.go

Lines 392 to 399 in 3583c1e

	// emptyLabels is a one-time allocation for non-partitioned metrics to avoid
	// unnecessary allocations on each request.
	var emptyLabels = prometheus.Labels{}
	func labels(code, method bool, reqMethod string, status int, extraMethods ...string) prometheus.Labels {
	if !(code || method) {
	return emptyLabels
	}

This map is later returned to collect the metrics from the context.

client_golang/prometheus/promhttp/instrument_server.go

Lines 100 to 102 in 3583c1e

	for label, resolve := range hOpts.extraLabelsFromCtx {
	l[label] = resolve(r.Context())
	}

This affects all promhttp versions