Refine the handling of OpenTelemetry resource attributes

[nosan]

Relates to #44394

Initially, I considered proposing these changes: main...nosan:spring-boot:gh-44394-1.
However, I eventually realized that it might be too much for this specific case.

This PR aims to eliminate duplications in OpenTelemetryAutoConfiguration and OtlpMetricsPropertiesConfigAdapter, consolidating everything into a single location.

[nosan]

Another option is to leave things as they are and simply add logic to ignore empty keys.

The attribute key MUST be a non-null and non-empty string.
Case sensitivity of keys is preserved. Keys that differ in casing are treated as distinct
keys.

main...nosan:spring-boot:44394

[mhalbritter]

Thanks @nosan !

[FyiurAmron]

@mhalbritter @nosan sorry for chiming in so late, but I found this in the current code:

	/**
	 * Default value for service name if {@code service.name} is not set.
	 */
	private static final String DEFAULT_SERVICE_NAME = "unknown_service";

 - shouldn't it say (similarly what it said before)

	/**
	 * Default value for service name if neither {@code service.name} nor {@code spring.application.name} is set.
	 */
	private static final String DEFAULT_SERVICE_NAME = "unknown_service";

 - because, based on the codes and later comments, both would be checked?

attributes.computeIfAbsent("service.name", (key) -> getApplicationName());
// ...
	private String getApplicationName() {
		return this.environment.getProperty("spring.application.name", DEFAULT_SERVICE_NAME);
	}

[nosan]

I am not sure I understood your question. The current arrangement is:

• If the user specifies service.name through management.opentelemetry.resource-attributes.service.name, it will be used.
• If the user specifies the environment variable OTEL_SERVICE_NAME, it will be used.
• If neither the environment variable nor the property is specified, then spring.application.name will be used.
• If spring.application.name is not set, then unknown_service will be used.

[nosan]

In simple terms, if service.name is not set by any of the above methods, "unknown_service" will be used.

[FyiurAmron]

@nosan that seems like what I would expect. The current DEFAULT_SERVICE_NAME javadoc is lacking in that case, precisely because it lost the 2nd part of the OR (previously, it was the only part that it had). Yeah, I know it's a private const, but people debugging stuff read those docs :)

BTW you probably read the previous version of my comment, I've edited it to make it obvious what I was expecting here.

[philwebb]

I've polished the javadoc in 22781fc