Matrix test for declarative config

[zeitlinger]

Is your feature request related to a problem? Please describe.

Even though the config bridge should make it transparent if declarative config is used or not, a matrix test would be good.

Describe the solution you'd like

We could run a matrix test with an env var DECLARATIVE_CONFIG=true that would

• assumeFalse() on unit tests or spring starter (not sure how to do that)
• collect all otel sys props on agent start and write them to a config file

The process is basically the reverse of

opentelemetry-java-instrumentation/javaagent-extension-api/src/main/java/io/opentelemetry/javaagent/extension/DeclarativeConfigPropertiesBridge.java

Lines 131 to 150 in df111a4

	private <T> T getPropertyValue(
	String property, BiFunction<DeclarativeConfigProperties, String, T> extractor) {
	if (!property.startsWith(OTEL_INSTRUMENTATION_PREFIX)) {
	return null;
	}
	String suffix = property.substring(OTEL_INSTRUMENTATION_PREFIX.length());
	// Split the remainder of the property on ".", and walk to the N-1 entry
	String[] segments = suffix.split("\\.");
	if (segments.length == 0) {
	return null;
	}
	DeclarativeConfigProperties target = instrumentationJavaNode;
	if (segments.length > 1) {
	for (int i = 0; i < segments.length - 1; i++) {
	target = target.getStructured(segments[i], empty());
	}
	}
	String lastPart = segments[segments.length - 1];
	return extractor.apply(target, lastPart);
	}

Depends on #14014 to get the test bridge (memory) exporter: https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/58a23223dad5e39f0c110a029cfce0c8cb397297/testing/agent-exporter/src/main/java/io/opentelemetry/javaagent/testing/provider/TestBridgeLogRecordExporterComponentProvider.java

Describe alternatives you've considered

No response

Additional context

No response