Prometheus attribute conversion should merge values when collisions occur

[jack-berg]

According to the prometheus compatibility spec, when converting attributes, if normalizing creates label conflicts the values should be merged together with a ; delimeter and sorted lexigraphically according to original attribute key name.

https://github.com/open-telemetry/opentelemetry-specification/blame/3a4cba26572558609bdcb51dfcbb2d8259085387/specification/compatibility/prometheus_and_openmetrics.md#L534-L537

We don't do this, instead using a naive "first wins" implementation:

opentelemetry-java/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/Otel2PrometheusConverter.java

Lines 462 to 524 in 9b602eb

	@SuppressWarnings({"rawtypes", "unchecked"})
	private Labels convertAttributes(
	@Nullable Resource resource,
	@Nullable InstrumentationScopeInfo scope,
	Attributes attributes,
	String... additionalAttributes) {
	List<AttributeKey<?>> allowedAttributeKeys =
	allowedResourceAttributesFilter != null
	? filterAllowedResourceAttributeKeys(resource)
	: Collections.emptyList();
	Map<String, String> labelNameToValue = new HashMap<>();
	attributes.forEach(
	(key, value) ->
	labelNameToValue.put(
	convertLabelName(key.getKey()), toLabelValue(key.getType(), value)));
	for (int i = 0; i < additionalAttributes.length; i += 2) {
	labelNameToValue.putIfAbsent(
	requireNonNull(additionalAttributes[i]), additionalAttributes[i + 1]);
	}
	if (scope != null && otelScopeLabelsEnabled) {
	labelNameToValue.putIfAbsent(OTEL_SCOPE_NAME, scope.getName());
	if (scope.getVersion() != null) {
	labelNameToValue.putIfAbsent(OTEL_SCOPE_VERSION, scope.getVersion());
	}
	String schemaUrl = scope.getSchemaUrl();
	if (schemaUrl != null) {
	labelNameToValue.putIfAbsent(OTEL_SCOPE_SCHEMA_URL, schemaUrl);
	}
	scope
	.getAttributes()
	.forEach(
	(key, value) ->
	labelNameToValue.putIfAbsent(
	OTEL_SCOPE_ATTRIBUTE_PREFIX + key.getKey(), value.toString()));
	}
	if (resource != null) {
	Attributes resourceAttributes = resource.getAttributes();
	for (AttributeKey attributeKey : allowedAttributeKeys) {
	Object attributeValue = resourceAttributes.get(attributeKey);
	if (attributeValue != null) {
	labelNameToValue.putIfAbsent(
	convertLabelName(attributeKey.getKey()), attributeValue.toString());
	}
	}
	}
	String[] names = new String[labelNameToValue.size()];
	String[] values = new String[labelNameToValue.size()];
	int[] pos = new int[] {0};
	labelNameToValue.forEach(
	(name, value) -> {
	names[pos[0]] = name;
	values[pos[0]] = value;
	pos[0] += 1;
	});
	return Labels.of(names, values);
	}

Found while reviewing open-telemetry/opentelemetry-specification#4963

cc @zeitlinger, @ArthurSens, @dashpole

[dashpole]

Huh, I implemented this a long time ago: #5717. Must have been removed :)

[jack-berg]

Yeah sorry about that. There was a rewrite (#6015) of the prometheus exporter to build it on top of the prometheus/client_java instead of handling the serialization ourselves. Must have lost this in the shuffle.