Prometheus label keys must not include colons

[dashpole]

What is the expected behavior?

From https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#metric-attributes,

OpenTelemetry Metric Attributes MUST be converted to Prometheus labels. String Attribute values are converted directly to Metric Attributes, and non-string Attribute values MUST be converted to string attributes following the attribute specification. Prometheus metric label keys are required to match the following regex: [a-zA-Z_]([a-zA-Z0-9_])*.

Metric names are allowed to include colons, but label keys are not.

What is the actual behavior?

SetMetricBasic translates attributes to prometheus labels:

opentelemetry-cpp/exporters/prometheus/src/exporter_utils.cc

Lines 261 to 276 in f52ec6c

	void PrometheusExporterUtils::SetMetricBasic(prometheus_client::ClientMetric &metric,
	std::chrono::nanoseconds time,
	const metric_sdk::PointAttributes &labels)
	{
	metric.timestamp_ms = time.count() / 1000000;
	// auto label_pairs = ParseLabel(labels);
	if (!labels.empty())
	{
	metric.label.resize(labels.size());
	size_t i = 0;
	for (auto const &label : labels)
	{
	auto sanitized = SanitizeNames(label.first);
	metric.label[i].name = sanitized;
	metric.label[i++].value = AttributeValueToString(label.second);

It uses the same sanitization function used for metric names:

opentelemetry-cpp/exporters/prometheus/src/exporter_utils.cc

Lines 131 to 139 in f52ec6c

	std::string PrometheusExporterUtils::SanitizeNames(std::string name)
	{
	constexpr const auto replacement = '_';
	constexpr const auto replacement_dup = '=';
	auto valid = [](int i, char c) {
	if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == ':' ||
	(c >= '0' && c <= '9' && i > 0))
	{

...which allows colons as a valid character.