Prometheus exporter: omit empty target_info metric

[dashpole]

Description

target_info should now only be generated when scope attributes and resource attributes are non-empty.

From open-telemetry/opentelemetry-specification#3660:

Prometheus exporters SHOULD generate an Info-typed
metric named otel_scope_info for each Instrumentation Scope with non-empty
scope attributes.

In SDK Prometheus (pull) exporters, resource attributes SHOULD be converted to
a single target_info metric
if the resource is not empty;

[olunusib]

Picking this up

[olunusib]

It seems this change doesn't need to be done in this repo @dashpole

[dashpole]

I believe we emit target_info even if there are no resource attributes:

opentelemetry-js/experimental/packages/opentelemetry-exporter-prometheus/src/PrometheusSerializer.ts

Lines 343 to 350 in f665499

	protected _serializeResource(resource: IResource): string {
	const name = 'target_info';
	const help = `# HELP ${name} Target metadata`;
	const type = `# TYPE ${name} gauge`;
	const results = stringify(name, resource.attributes, 1).trim();
	return `${help}\n${type}\n${results}\n`;
	}

[dashpole]

I've updated the description to be more accurate, as otel_scope_info changes are not needed here.

[olunusib]

I believe we emit target_info even if there are no resource attributes:

From the code below, the default resource is always used if no resource is specified (or an empty one is passed in) so we'd never have an empty resource.

opentelemetry-js/packages/sdk-metrics/src/MeterProvider.ts

Lines 47 to 57 in f5ef8de

	constructor(options?: MeterProviderOptions) {
	const resource = Resource.default().merge(
	options?.resource ?? Resource.empty()
	);
	this._sharedState = new MeterProviderSharedState(resource);
	if (options?.views != null && options.views.length > 0) {
	for (const view of options.views) {
	this._sharedState.viewRegistry.addView(view);
	}
	}
	}

If we choose not to emit target_info when the default resource is used, it could break the existing API, and I cannot predict the extent of the impact.

[dashpole]

Thanks for clarifying!